Hikari (Kari) Yagami in Digimon Adventure, younger sister of Taichi (Tai), who gets the Incurable Cough of Death during one episode. Through a flashback in the anime, she is shown to have gotten pneumonia. It's later stated that Hikari had not properly healed from a huge cold that left her bedridden and unable to join the kids who'd become the Digidestined in their fateful school trip. Said cold came back with a vengeance when they were in the Digital World, causing Hikari to fall down with a fever that was just as bad as back home — only that in this particular Sick Episode, they had no ways to properly treat her, and for worse Machinedramon was tracking them. It triggered Taichi's bad memories of the incident in which, years ago, he accidentally caused Hikari to almost die of pneumonia.
Relena Norstein from Digimon Savers, which led to her older half-brother Touma joining sides with Kurata to get her "cured." This is a prime example of the writers not defining the disease: she's in a wheelchair, although she can walk short distances, and Touma wins the Nobel Prize for curing her, but it's incredibly vague otherwise.
Chiho, Uzume's Ashikabi from Sekirei has a mysterious unnamed illness that keeps her hospitalized. It later becomes a pivotal plot point towards the middle of the second season (Pure Engagement)
Rin (stomach ulcers coming from her Broken Bird personality and the stress of her horrible former home life) and Akito (unclear illness; maybe the same sickness that killed her father, or maybe psychosomatic). Even though they're sick, they both scare the living crap out of people: Rin because she's headstrong and harsh, Akito because she's the feared leader of the Sohma clan.
An episode of Galaxy Angel has an ill Mint in a hospital while watching the leaves on a tree — she hopes to live long enough to watch a leaf fall off — and it promptly does, at which point she notes she was referring to another leaf, which also falls off. As she tries to refer to a third leaf, all the leaves are blown off by a strong wind. Another character, Chitose, is an obvious parody, claiming to have been confined to a hospital most of her life. More likely it's another product of her attention whoring; she was enough of a Scrappy in the game that even the anime writers didn't like her, and her personality was exaggerated to that of a wangsty histrionic (although to be fair, everyone else was exaggerated too).
Hayate Yagami in Magical Girl Lyrical Nanoha A's. However, in the sequel series StrikerS, she is completely healthy. This because her "illness" was actually the curse of the Book of Darkness, so once the Book was destroyed she was able to recover.
Hyatt in Excel♥Saga is a particularly over-the-top parody of this, as she tries to go about her duties as an agent of ACROSS despite her tendencies to collapse, cough up contaminated blood (which kills small birds and sickens humans), faint, or even drop dead and then spontaneously come back to life. She always comes back to life, once even being charred to a decent crisp only to sit up later, as if nothing had happened.
This appears to be an empathetic illness, at least in the manga. For several volumes Hyatt went some time without her usual blood and death routine, but the revelation that the competent Excel was an impostor brought it back.
Sailor Moon R: Anti Villainess An occasionally fell into the archetype, getting tired and out of breath easier than her "brother/lover" Ali/Seijuuro. Although in An's case this was caused by her starving, since she refused to eat regular food like Ali did, and humans' life energy wasn't available every time she needed it.
Hotaru Tomoe, who suffers seizures and cannot make huge physical efforts. She more than once ends up collapsing in the streets.
One-time character Misa-chan from the Stars season, an hospitalized little girl who bonds with Taiki/Sailor Starfighter.
Megumi aka Megu, Suigintou's medium and Morality Pet from Rozen Maiden. She has extremely low self-esteem as a result, and Suigintou sorta empathizes with her.
Angelic Layer: The mother of the main character, Shuuko, is an adult Ill Girl who abandoned her daughter because she "didn't want to be a burden" and feared Misaki would be ostracized for having a "useless" mother. This gives her somewhat of an excuse for literal Parental Abandonment, leaving Misaki in the "more capable" care of her aunt. (Meanwhile, the entire Angelic Layer game is an elaborate plan coming from her friend Icchan to get enough data on human neurophysiology to cure this disease.) Curiously, this is not in the manga at all.
Code Geass: Nunnally Lamperouge/vi Britannia, the younger sister of the main character, is both blind and contained in a wheelchair after she witnessed and barely survived their mother's brutal murder. He promises her that by the time she gets well, the world will be a better place. He never mentions how — to him, it doesn't matter how many people have to die or be manipulated to destroy the The Empire. Further, she didn't actually see it, having been a false witness due to a memory-altering Geass. Also, she gets better with time: in the series, she stays in a wheelchair but regains her sight and becomes the Empress of Britannia, and in the Nightmare of Nunnally manga she heals completely and stays under the wing of Empress Euphemia Li Britannia.
Also, fellow student and revolutionary Kallen Stadtfeld/Kozuki uses the image of the Ill Girl to cover up her frequent absences from school so she can go play revolutionary. Maintaining the illusion limits her somewhat, but fortunately there's usually a distraction around to keep others from asking when she makes an unexpected display of physical prowess. Lampshaded in the 2nd picture drama, where Shirley grabs a naked Kallen from behind while they're bathing and comments on how Kallen is much stronger and athletic than she's supposed to be.
Kikyou took this role more than once, despite being undead. Namely, it happened every time she was either weakened by poison or suffering the physical downsides of her clay body.
Though it's never mentioned in the anime, Kurz Weber of Full Metal Panic! is stated to have joined Mithril in order to pay the hospital bills of a young girl who was critically injured in a mission he was involved in. More exactly, during the assassination of the man responsible for the deaths of Kurz's parents. Kurz, who was supposed to take the shot, stopped when he realized that the poor girl would also be hit; his Cold Sniper mentor, Wilhelm Casper, had fewer scruples and fired anyhow.
Anju Kitahara in the anime version of Marmalade Boy. She has a weak heart and might not live for long if she doesn't go through a very dangerous operation in the USA, but she attempts to remain cheerful and befriends her beloved Yuu's girlfriend Miki, telling her that she won't break them up and is as happy as she can with being Yuu's Unlucky Childhood Friend. She's operated and gets better, and when Yuu breaks up with Miki she has a small chance to get Yuu for herself, but ultimately she (and Miki's other love interest Kei) give up when they get back together.
Toji Suzuhara's off camera sister in Neon Genesis Evangelion: After the injuries she suffers among the civilians caught in the middle of the fight in the first two episodes, Toji is eventually recruited as the pilot of Unit 03 in exchange for her being transferred to NERV's medical facilities. However, Toji himself is severely crippled when an Angel takes over his Eva.
In Rebuild, we get to see Toji doting on his little sister as she finally gets out of the hospital, and Asuka ends up piloting Unit 03 in his place.
Otohime Mutsumi from Love Hina, who has a severe case of anemia and passes out almost everywhere. During her Crash into Hello with Keitaro, she fell over, with Blank White EyesandBlood from the Mouth. Keitaro checks for a pulse...and there isn't one. Cue Freak Out. She got better, but it seems her fainting spells are closer to actually dying then "normal" fainting. Read about it here.
Naru used to be an Ill Girl as a child. She has asthma, and while she's mostly fine in teenage/adult years, in her childhood her attacks were much more serious. Several years ago, Naru's parents sent her to Hinata Sou hoping that she'd recover due to the zone's benign climate, but she was very lonely and depressed. That was the time when she befriended two older kids, Keitarou and Mutsumi, and the three made The Promise.
Fujimiya Aya fills this role for most of Weiß Kreuz after she's hit by a car and ends up comatose, prompting her older brother Ran to become an assassin to pay her hospital bills—and to actually assume her name in tribute. She ultimately gets better.
In one episode Ran encounters another Ill Girl, whose brother has entered a Deadly Game to pay her medical bills.
Mitsuki Koyama from Full Moon o Sagashite has a cancerous tumor that prevents her from singing, and gives her exactly one year left to live. When she transforms into "Full Moon", she temporarily loses this tumor, and gains an amazing singing voice.
In Binbou Shimai Monogatari Asu winds up in hospital from a neglected cold, giving her sister Kyou the chance to indulge in emotional memories about the time their mother was hospitalized.
Rico turns out to have been an Ill Girl prior to becoming a cyborg, languishing away in an hospital after her parents simply abandon her there to her luck until she's taken in by the organization; meanwhile, Angelica eventually becomes one due to the side effects of the "conditioning" the girls go through. Eventually though, all girls will meet the same fate, since their lifespan is immensely shortened due to their cybernetic implants.
Also, Claes stops taking roles in missions not just because of her trainer Raballo's death, but because she actually has a weak heart.
Elisabeta, a Chernobyl survivor and former aspirant ballerina who developed cancer and tried to commit suicide after her cancerous leg was amputated. Then she was brought into the Agency and became Petrushka aka Petra.
Parodied in Magical Project S, where one of the Love-love Monsters is called "fake sick girl", who even gives the speech about not seeing the last leaf fall from a tree. When the person talking with her mentions that the tree is quite full and healthy, she blows it up with a cannon hidden in her bed.
Subverted quite quickly in The Daughter of Twenty Faces, in that the "ill girl" is in fact the main character who recovers quite quickly when taken away from the source of her "illness" by the eponymous Gentleman Thief Twenty-Faces.
Usopp's best friend Kaya from One Piece, a very delicate young woman who goes through quite the Break the Cutie. She got better later, though.
Dr. Black Jack had five patients of this type in the new anime series:
Pinoko, his adopted daughter assistant. She was originally a sentient tumor in the body of a young girl, whom he extirped and gave a doll-like body out of pity since he found out that she strongly wished to live, just like Black Jack wanted to after the accident that killed his mother and left him paralyzed for years. Pinoko's sister had a Freak Out and disowned her, so Black Jack took the kid around as his adoptive daughter and assistant.;
Michiru, a Teen Genius manga artist whose biggest dream was to get married before she died, so Black Jack played the charade to make her accept the operation that would save her, later "breaking up" with her so she wouldn't let her one-sided love for him stunt her. She not only gets better, but ends up marrying her Unlucky Childhood Friend.
Watou and Kumiko's classmate Rei, an Idol Singer who takes her career as reallySerious Business, but has a tumor in her throat that will put it in serious risks.
Megumi Kisaragi, a female doctor whom Black Jack fancied, but had advanced ovarian cancer. After having her affected inner organs removed, she changed her name to Kei and left, living her life as a male, despite not being outright said if she went through a sex change or not.
In the backstory, there is another: Black Jack's own mother, who was severely injured in the same incident that left him as an Ill Boy and died few months later.
Yutaka, Konata's paternal cousin, has recurring (albeit mild) health problems that make it difficult for her to keep up with her friends— she was nearly rendered a Broken Bird from the social isolation caused. And her case of Older than They Look is even more severe than Kanata's...
Parodied and played melodramatically in Mamoru-kun ni Megami no Shukufuku wo. During the Valentine's Day episode, a contest is on to see who can give Mamoru chocolate in the most creative way possible. Shione orchestrates a "Last Leaf" situation with her brother claiming her lack of fashion sense is a disease affecting "one in six billion people" that will kill her. In the end, Shione "dies", but not before giving Mamoru her chocolate...shaped like her hairstyle.
Yuuki Konno from Sword Art Online. She contracted AIDS when she was born. By the time she was introduced in Mother's Rosario, she was already blind, bedridden and on the verge of death in the real world.
Sora from Mahou Tsukai Ni Taisetsu Na Koto Natsu No Sora seems to be mostly in good health, having little trouble with physical exercise or performing straining magical rituals—until she suddenly appears to have been suffering from a terminal heart condition for a while already, leading to a severe mood whiplash in the series.
Four Murasame from Zeta Gundam, a war orphan turned into Tyke Bomb who got lots of power via treatment and training, at the cost of terrible mental instability and constant physical pain (in Four's case, splitting headaches that affect her more than once).
Played with in Gundam X. Female lead Tiffa Addil is a Fainting Seer and kinda delicate, but this doesn't stunt her that much. Her only brush with death was actually a side-effect of her being poisoned by one of the villains.
Stella Loussier from Gundam SEED Destiny. Being one of the Extended, her life hangs on by treatment in the Alliance medical rooms and she will die if she doesn't get it, which is shown in her seizures and memory gaps. For worse, the training has damaged her mind as well, and she's few more than a very childish Ophelia almost unable to function in real life when outside missions. Yeah, it sucks to be her.
Gundam 00: Louise Halevy in the second season. To start, Louise lost her left hand in the bombing incident that decimated her family in the first season. She got said hand back through an undisclosed medical treatment and Took a Level in Badass that led her to become a Dark Action Girl, but she also had some very ugly seizures and had to pop some pills. Her sanity continued slipping as the series advanced, but luckily, by the end she starts getting better. In The Movie, she's mentally doing better but still in physical recovery.
Lu Anon from Gundam AGE is a tragic non-combatant example. A Vagan girl who is Kio Asuno's age, she suffers from a terminal disease due to exposure to Mars Rays. In the little time she has left, she wants to live happily and normally, especially after she meets Kio. The time they spent together and her heartbreaking death became major turning points in Kio's attitude towards the enemy and the war.
Someone subverted in Planetes, where during a hospital stay Hachimaki meets a girl who he believes to be chronically ill, but does not want to be rude by asking. In reality, she is a child who was born on the moon, and is staying in the hospital to provide medical information to scientists and doctors.
Mahou Sensei Negima! has Ako Izumi briefly filling the role after she catches some sort of disease upon arriving in Magicus Mundus. It varies a little bit in that she gets the cure early on, but it's so expensive that it forces her and two of her friends (Natsumi Murakami and Akira Ookochi) to sell themselves into slavery. This in turn leads to Negi having to enter a tournament to win the money to buy their freedom.
In the Cowboy Bebop episode "Waltz for Venus," Spike befriends a smuggler named Roco who is trying to earn enough money to pay for his blind sister Stella's eye operation. It ends in tragedy.
Kotori Monou from X1999 has a serious heart condition. One of her older brother Fuuma's first scenes in the manga has him reading out loud a list with the remedies that Kotori has to take that day.
Deconstructed in a Detective Conan case: Our Villain of the Week had a bad heart, but it was her who killed the loanshark who drove her boyfriend to suicide over the money for "that operation" (see Finger-Licking Poison for the murder itself). God, is she a bitchy chain smoker. In her confession, she mentions that her boyfriend was her Morality Pet and the only one who ever loved her after her parents's deaths. Be careful when threatening an Ill Girl's loved ones: if she gets better, she'll kill you!
Played heartbreakingly straight in the fillers, with a girl named Kaori whose older brother Todakuro is the ghost writer to a murdered novelist, Daisuke Torakura, who paid for her hospital bills in exchange for the guy's hard work. We later learn the horrible truth: Kaori actually died a year ago because Torakura actually bribed her doctor into keeping her on painkillers only instead of paying for crucial and expensive treatment abroad (as per his end of the bargain), so Todakuro would remain indefinitely time as his subordinate, which ultimately killed her. Then, Torakura maintained the lie via keeping Todakuro locked in his countryside villa, telling him that Kaori (supposed to be in an American hospital) was going to get better. When poor Todakuro found out the truth from the guilt-ridden nurse who used to tend to Kaori, he went nuts with pain and murdered Torakura. Asshole Victim seems a bit lenient... Basically, Torakura was Moving the Goalposts.
Subverted quite cruelly with Aiko from the Vampire Princess Miyu OAV, who was thought to be in a Convenient Coma due to either illness or Demonic Possession, but Himiko later discovered that she summoned the main Shinma in despair after blaming herself for her parents's death, having survived the accident that caused their demises. Ever since then, she lays on her old futon located in her her decadent Big Fancy House and simply... sleeps, while the Shinma wreaks havoc around. She only wakes up when Himiko confronts the Shinma, but Miyu stops her when she's about to drink Himiko's blood, and once the Shinma is defeated Aiko dies.
In the Miyu Spin-Off series of sorts Vampire Princess Yui, this is also subverted. It's believed that Yui is an Ill Girl with severe anemia, but the truth is that her Guardian powers have just awakened and, since she had no real idea of what was going on, she lacked of the blood "meals" she needed. (It's not helped by how Yui at first refuses to drink blood from humans if she can help it, so her Battle Butler Nagi gives her his blood instead.)
Though no one takes her seriously, Umino of Satou Kashi no Dangan wa Uchinukenai claims that her painful limp and clumsiness were caused by a witches spell that turned her into a human with an imperfect pair of legs. She says this same spell will turn her into sea foam in one months time if she does not fulfill her wish of finding a "precious friend."
It turns out she's been abused by her father for years. Her limp is due to a hip injury received as a baby, and to top if off she is deaf in one ear due to her father as well.
Very cruel version: Miyuki Aiba/Shana Carter aka Tekkaman Rapier/Teknoman Shana from Tekkaman Blade. She's actually a Tekkaman whose transformation process was incomplete, therefore she suffered horrible pains and her lifespan was dramatically shortened... therefore, she chose to go down fighting and perform an Heroic Sacrifice than die of her illness.
Tsubaki Kakyouin aka "Tsubaki-hime" and Muraki's fiancée Ukyou Sakuraiji, both from Yami No Matsuei. We have next to none clues on Ukyou but we know that Tsubaki has an extremely weak heart, which led Muraki and other people to kidnap her Only Friend Eileen and then kill her to harvest her heart. When poor Tsubaki learns about this, shedoes nottakeitwell.
Bleach: Byakuya Kuchiki's deceased wife, Hisana. It's implied that her sickness was caused or compounded by her repetitive guilt-driven visits to Rukongai in search of her lost sister, Rukia.
Inori's older sister Seri in Harukanaru Toki no Naka de; Inori even has the Big Brother Instinct towards her because of this, despite him actually being younger. A flashback in the Hachiyou Shou TV anime reveals that this is part of the reason Seri got close with Iktidar, who saved her when she was threatened by several men while helpless from her condition. Somehow, being an Ill Girl didn't stop Seri from having Ikutidaru's baby.
Anemone from Eureka Seven. This Dark Action Girl has to live drugged to get away with the process she underwent to become Coralian-like, which causes her horrible side-effects like headaches, psychic nosebleeds and wild mental unstability. The doctor even says her resistance to the drugs was her greatest value.OUCH!
Female lead Eureka, as well. She spends much of the first half of the series feeling progressively worse due to her diminishing connection to the Nirvash.
Takiko Okuda and her Missing Mom Yoshie from Fushigi Yuugi Genbu Kaiden. Takiko caught tuberculosis from the deceased Yoshie while tending to her in her own world. She insists it's just a cold, as her father makes plans to take her back to Tokyo for medical treatment.
Kozue Ayuhara from the old-school anime Attack No. 1 is ill and has to quit sports awhile and move where she can rest because of it, until she comes back and joins the volleyball team.
Kosumo Koganehara from Michiyo Akaishi's Towa Kamo Shirenai was born with a a weak heart, and in fact the manga starts when she has a seizure during her treatment in the USA and needs an urgent transplant, but her extremely rare blood type doesn't allow for an easy one. The donor that saves her life just happens to be Himiko, a Miko and Magical Girl Warrior who has just died via an Heroic Sacrifice to save a child, so Kosumo inherits her powers alongside her heart and blood and must take up her demon slaying mission - with the help of Himiko's Tall, Dark and Handsome guardian, Hitsuji.
And later we meet another: the 98th Himiko, the one who came before the aforementioned Himiko we know. She was a fellow Miko who died of cancer when the 99th Himiko was 10 years old, which wasn't helped by her already suffering of a years-long depression and being separated from her bodyguard and lover Hishou.
Katekyo Hitman Reborn!: Chrome Dokuro/Nagi will die if Mukuro or Daemon Spade isn't giving her illusionary organs to replace the ones that she lost when she was hit by a car.
In the second season fo Get Backers, one of Ban and Ginji's missions is to get a very rare type of blood (named the "Bombay Blood") for an Ill Girl named Yumiko, who needs an urgent transfusion or she'll die in few days. It turns out that Akabane has a dose of it and they have to fight him and his partner Maguruma. It's also revealed that Ginji had an Ill Girl friend in the Infinite Fortress days and she died due to lack of treatment, so he's determined to not fail again for Yumiko's sake. They get the Bombay Blood, and Yumiko is saved.
Homura Akemi from Puella Magi Madoka Magica, before meeting Madoka and her friends, was this. She spent most of her life in the local hospital due to a severe heart illness. It's not specified how she got rid of it, but most likely did it with her Magical Girl powers. This is also the basis of her attitude towards Madoka, as her social isolation and orphanhood means the latter is the first and only friend she could made for the past 14 years.
Also, according to popular Fanon, the witch Charlotte may have been one when she was still a Magical Girl. What prompted this Epileptic Tree was that her Grief Seed first appeared outside a hospital, there is a general medical theme to her barrier as well as the sweets, and the fact that she's obsessed with cheese, which chemotherapy patients aren't allowed to have. Add in the detail that Kyubey has contracted girls on the verge of death at least once (see: Mami, who had been seriously injured in the car crash where her parents died), and this theory became popular.
Yuki Miyata from Moshidora. Her illness is not specified, aside of being a congenital one; her hospitalization requires her best friend Minami to take her place as the local Cute Sports Club Manager. Unfortunately, Yuki does NOT get better, and Minami suffers an Heroic BSOD when she dies.
Alitta from the Netherlands arc. Fortunately, it's a temporary illness rather than a life-threatening one. Unfortunately, she's a famous stage actress and thus she needs her Identical Stranger Lunlun to be her temporary Body Double...
Lucero, the little girl from the Orphanage of Love in the Italy arc, who's blind. Her friends decide to search for an old treasure buried near the orphanage to sell it and get the money for the orperation. The treasure is actually an old bomb.
Haruna from Tenchi Forever! is this. After fleeing with her lover, Yosho/Katsuhiko, she became ill during their trip. She died before the couple arrived on Earth.
Ultear Milkovich from Fairy Tail was this in the past. Then she fell in the hands of a bunch of Mad Scientists whom her mother asked for help, not knowing about their intentions; they did heal the little girl, but they told Ur that she had died and then told their ex-patient that her mother had abandoned her... just so they could make the desperate Ultear their Dark Action GirlTyke Bomb.
Kaoru "Kaoru-no-Kimi" Orihara from Oniisama e... was held back a year due to an unspecified illness, and when the story begins she has just returned to Seiran Highschool. As such, she not only hates it when people neglect their physical well-being, but sometimes collapses in fits of pain and needs both rest and painkillers. What's her ailment? Breast cancer, and she even had to go through a mastectomy at age 16, which caused her to break up with her then-boyfriend Takehiko (the titular Oniisama), and when she returned, she kept her illness a secret from most of her classmates until the symptoms of a possible relapse were shown towards the end. What happens to her varies according to the different continuities: In both of them she goes to Germany with Takehiko after they rekindle their relationship... but while she lives on and they stay Happily Marriedwith a child in the anime, in the manga Kaoru does die two years after leaving Japan.
Three show up in Honoo No Alpen Rose: Clara (temporary but plot-important illness that has her brother rat out Jeudi and Lundi to get enough money for her medicines), Marie ( confined to a wheelchair, but starts to walk after befriending Lundi, and specially Jeudi's Missing Mom Helene (weak heart and almost total blindness after the accident that separated her from Jeudi.)
It's horribly subverted in the case of the third, too: the villain Toulonchamp and his daughter Mathilda have been slowly poisoning her for years so they force her parents to collaborate in their schemes.
Asa Shigure from SHUFFLE! suffers a mysterious illness that often leaves her bedridden, and starts becoming worse as time passes. It's not an illness, but Power Incontinence: her weak half-demon body cannot handle the strong magic inherited from her mother, the demon Ama, but Asa utterly refuses to use said power because Ama suffered horribly due to them, thus Asa doesn't want her mother to be sad. Rin has to take a VERY risky third option to force her release said energy, and almost dies for it.
Kiku from Idaten Jump, also Kouhei's little sister and the local Yamato Nadeshiko. Her illness turns out to be a consequence of the disbalance between the X Zone and Earth, and once it's fixed she starts to get better.
Chizuru Nanbara from Combattler V. Her illness is explained (malformed heart valve), it kicks in at a critical moment (during battle!), and while it keeps her away from action for a while, she's successfully operated on and returns to the battlefield.
There were four in Angel Beats! when they were still alive: Masami Iwasawa (suffered a stroke after being hit on the head by her abusive father and died in the hospital), Yui (was quadriplegic after an accident and languished away in bed until she kicked it), Otonashi's dead little sister Hatsune (undisclosed illness that ultimately kills her) and Kanade "Angel" Tachibana (heart illness that leads her to need a transplant). In fact, after the latter died, her main goal was to find and thank the person who donated their heart to her and allowed her to live a little longer... Said person turned out to be the male lead Otonashi.
The title character of Binetsu Shoujo ("Feverish Girl"), Rina Kisaragi. She has anemia and tends to pass out quite a bit, so she often goes to a local hospital for check-ups.
Toki Onjouji and Yuu Matsumi from Saki Achiga-hen. The former has a tendency to faint at any given moment, and the latter gets chilled very easily to the point where she has to wear a cardigan, mittens, and a muffler even during the summer.
Leonora Fenton in My Daddy Long Legs seems to have classic symptoms of tuberculosis of the lungs. It's actually a heart condition that gets corrected by surgery.
Lynn's half-sister Sarah Russell from Lady has always had a very frail health. She has to be homeschooled since she often spends her days feverish in bed, and once in a while passes out if she's too stressed.
Lita is a girl who is in a wheelchair whom Hayato meets in the Peruvian GP. She recently has surgery for her legs, but she's afraid to use them despite them being completely healed. After the race, she gets up thanks to the courage he gives her.
Elena is the daughter of the orangizer of the Fireball Race and has a weak heart since birth. During the race, she catches a fever while navigating with Hayato and as her fever worsens, he calls in HSR-III to take her to the hospital. She gets better at the end of the episode.
After's Kamille Bidan Heroic BSOD at the end of Zeta Gundam as a product of Scirocco's massive Mind Rape, he remains in a vegetative state throughout the sequel, Mobile Suit Gundam ZZ. The few times he's more or less awake, he acts very erratically and occasionally uses his Newtype powers to call out to other characters such as Judau. He successfully recovers at the finale.
In Ayashi no Ceres, Chidori's little brother Shota is hospitalized when introduced.
Captain Jushiro Ukitake from Bleach may fit this trope. He's been suffering from tuberculosis all his life, which makes him spew blood if he exerts himself too much, turned his hair white, and causes him to spend most of his time in bed. (Although since he's over two thousand years old, it's obvious he isn't dying from it... Unless he's dying very, very slowly).
Oddly enough, Mai Tokiha's brother Takumi from Mai-HiME and Mai-Otome manages to fit this trope while also being a domestic type (at least in the former show), and is inordinately fond of his older sibling.
A further subversion is that Yukimura does in fact have a specific illness (Guillain-Barré syndrome, a disease of the nervous syndrome) which led to some fan complaints after he recovered, because this condition is incurable. It's later handwaved in an author's note, apparently: he does not suffer from Guillain-Barré, but from a similar illness... which is not mentioned by name.
Since this is a series about sports, there aren't many boys with bad healths but their "places" are generally taken by characters who have serious game breaking injuries at some points and cannot play at full potential, lest their injuries will get worse and put their careers at risk. Some examples of this subversion are: Wakabayashi and the aforementioned Misugo in the first part of the manga; Wakashimazu and specially Tsubasa himself in the second part of the manga and in Shin; Wakabayashi again, Misaki and the South Korean captain Cha Inchon from the World Youth Cup manga.
Rosette Christopher's brother Joshua in Chrono Crusade was plagued by constant seizures when he was a child — made even more frustrating by the fact that they came alongside healing abilities that were completely ineffective on himself. It's this that drove him to accept Chrono's horns from Aion, completely unaware that they would drive him completely insane.
Played with in UFO Robo Grendizer. Duke Fleed/Daisuke Umon is an Ill Boy who's actually dying of radiation poisoning... and the lead character
An ill baby boy is Ivan Whisky aka Cyborg 001 from Cyborg 009. His enhancements somewhat cure or slowen his illness, but he's now trapped in the body of a baby forever. On the other hand, he's now a very powerful psychic.
Akira Sohma, Akito's father in Fruits Basket. He had an unnamed illness, which may have been compounded by the stress of being Head of the Family, and by the time the series starts he's been dead for years.
Yuki was an Ill Boy when he was little, too, as evidenced by his persistent cough. It's mainly just asthma, though, which he eventually outgrows.
Luna's sort-of temporary love interest in the movie (and in a side-story of the manga) was an ill boy Hot Scientist named Kakeru Ohzora. The MacGuffin of the movie (a crystal belonging to the Big Bad, Princess Kaguya), which is in his possession, saps his Life Energy away as the movie passes - he starts relatively healthy, then starts to cough and later is weakened and bedridden. He gets better
Mamoru Chiba spent the last part of the Super S season as an ill boy as the Dead Moon Circus takes over ther Earth more and more, which causes him great physical pain and weakness since, as Prince Endymion, he's the equivalent of Sailor Earth. When the Circus is banished away, he gets better.
Kenneth "Ken" Robbins from Kaleido Star. His dream was to participate in the Kaleido Stage as one of the acrobats, but since he got a weak heart, he mostly does staff job. Later in the series, he does some acting as well, but not with any life-risking acrobacies.
Subverted in Code Geass R2. Lelouch's opponent and later ally Li Xingke has an Incurable Cough of Death and might not live for long, but remains an excellent pilot and strategist. He makes it to the end of the series, but it's hinted that he may have passed away after the Zero Requiem.
In the original series we have Kenshin and Saitou's old rival Souji Okita, the most badass member of The Shinsengumi — who also has tuberculosis and is seen coughing his lungs off. Just like Takasugi, he was based on a real life figure affected by such an illness. And again, just like Takasugi and his own RL counterpart, Okita died before the Restoration.
Romeo's commoner friend Petruccio from Romeo X Juliet, afflicted with what's all but stated to be turberculosis. He dies in Romeo's arms in the same episode he's introduced.
Subverted and parodied mercilessly by the manga-only character of Densuke in Ranma 1/2. He is a sickly, bedridden boy who muses that he will die by the time the last persimmon falls from the tree outside his window... cue female Ranma coming by with a huge sack to pick all the persimmons herself. Densuke's illness is never specified, however, he is relatively healthy (if abnormally weak) and all it would take to cure him is a single dose of a powdered medicine. He refuses to take it, though, unless delivered mouth-to-mouth by a pretty nurse.
Parodied earlier in both the Manga and the anime by the "ill boy" who insisted on keeping Genma (in his Panda form) as a pet. It turns out he isn't sick, just lazy (and spoiled.)
Eisuke Hondou's important backstories were all when he was hospitalized; first due to leukuemia and then due to accident. In the first case, he was saved due to his older sister's marrow donation, which changed his blood type and tipped Conan/Shinichi off in regards to said sister's own identity and role.
Seiji Asou from the Moonlight Sonata case used to be this as a young boy. In fact, when he was hospitalised in Tokyo, his family (including his father, a famous pianist) was killed in strange circumstances, related to the authorities from the island he grew up in and the shady dealings they had with his dad. For the rest of the story, see Harmless Lady Disguise.
In a filler case, the Detective Boys and Conan go to an hospital to visit their classmate Jun Kataoka, who is so ill that he'll have to stay hospitalised for weeks. Since Jun is also Conan's Identical Stranger, at the Detective Boys's suggestion they swap clothes and places for a day. And then Conan is abducted by a mysterious man who wants to blackmail Jun's very rich widowed father and his soon-to-be stepmom, but he manages tp escape. And then, the kidnapper appears dead...
The whole sixteenth movie, The Eleventh Striker, had one of these as motivation: The Big Bad and Anti-Villain was a man who had an Intergenerational Friendship with an ill boy named Tomofumi Motoura. Poor Tomofumi who died years ago when he was in need of medical attention but a soccer mob stopped the ambulance he was in and prevented the medical staff to save his life. (And a drunk Kogoro was a member of said mob, too.) So the Big Bad became a Mad Bomber hell-bent on revenge against both Kogoro and anything related to soccer...
In the Magic Kaito special Tears of Love for the Black Knight (based on a story from the fourth manga volume), FBI agent Jack Connelly's son Kenta is one of these, and he needs surgery to get better. His father is so desperate to get enough money that he becomes a thief named Nightmare so he can gather cash and Kenta can be operated on. At the end of the special Kaito Kid confronts Jack on how not only he's a "newbie" in regards to classy thievery, but on how this whole deal will badly hurt Kenta if he ever finds out, and attempts to get Jack to quit... It endsin tears.
Lasse Aeon from Gundam 00 was healthy in the first season, but in the second one he had quite the Incurable Cough of Death. It's apparently a consequence of his battle with Alejandro Corner.
Dennou Coil, which seems to enjoy gender-flipping a number of traditional shonen anime roles, has a sick boy in the form of Harakawa Kenichi aka "Haraken." He suffers from fainting spells and some kind of poorly-defined heart flutter that may or may not have to do with his investigations into the mysterious Illegals.
England takes up the role briefly in the episodes featuring The Grim Reaper.
Spain, when his economy is in a terrible state and he gets a cold because of it.
According to Word of God, Austria spent some years in a wheelchair. It's not explained when this happened, but fanon speculates that it may have taken place either after the dissolution of the Austro-Hungarian Empire or right after World War II.
Dr. Kuro Hazama aka Black Jack, thanks to the hidden field mine that went BOOM on him and his mom, killing Mrs. Hazama and severely injuring him. Of coruse, he also attends many ill boys and girls as well; in fact, regular character Sharaku was introduced as one in the newest series, since he and his sister Wakou join the cast after BJ has to operate on him in the first episode.
Masataka's little brother Mitsugu from Sakura Gari. He was born with a weak heart, barely survived Spanish influenza in the past, and in-series he gets pneumonia. Ouch.
Kyousuke Kamijou from Puella Magi Madoka Magica was a talented violinist involved in an accident, thus we meet him in the hospital and see that not only his arm was injured to the point that he won't be able to play anymore, but he must go through painful therapy to regain use of his legs. The manga goes further by directly showing the horrible scars on his arms. Sayaka, Madoka's best friend who fancies Kamijou, becomes a Magical Girl and uses her wish to heal him so he can play again. Too bad that, in this particular world, making a Selfless Wish is not a good thing.
Sho from The Borrower Arrietty. He tells Arrietty he has always been ill and can't play with other kids. The reason he is currently staying at his aunt's place is because he is getting an operation on his heart.
Friederich Brandel from Honoo No Alpen Rose aka Jeudi's Disappeared Dad. In a subversion, a good part of his horrible health comes as a consequence of a terrible accident he was involved into... and he dies of it, right after having been reunited with Jeudi.
Michio Yuki from MW. Not only the titular chemical warfare turned him into an asshole beyond redemption, but also gives him occasional headaches, getting hospitalized at one point, and only a few years left to live.
Yayoi from Loveless, which seems to be the result of severe athsma.
HeartCatch Pretty Cure! has a very interesting case — Itsuki Myoudouin's older twin brother Satsuki is a case of Ill Boy and a Heir to the Dojo. With him sick, Itsuki's determined to take over their grandfather's dojo, leading her to become a Wholesome Crossdresser because she thinks she won't be taken seriously otherwise. It isn't until halfway through the series that Satsuki goes through an operation and is cured (though it's a pretty risky surgery that could've potentially killed him), leading to Itsuki to start abandoning her role, openly embrace her love for more girlish things and be the Third Precure.
Three male characters in Another have this. Our protagonist Kouichi Sakakibara has pneumothorax (collapsed lung),and he's hospitalized due to it for almost a month at the start of the story, making him miss his first day at his new school and thus missing several details about what will go on; Ikuo Takabayashi has a heart conditionand it takes away his life, and Daisuke Wakui is asthmatic.
Coach Jin Munakata from Aim for the Ace!. We know what he has: leukemia. And unlike others, he actually dies.
Nikaidou from Sangatsu No Lion. He has an unspecified illness that often manifests in the form of anemia. It's frequently mentioned by members of the cast, and is something of a minor sub-plot involving him.
In Bokurano, Kunihiko Moji's best friend Nagi is an ill boy with a weak heart and is hospitalised when the plot rolls in. Moji, who has signed up to pilot a Humongous MechaPowered by a Forsaken Child, wants his own heart to be harvested and given to Nagi for a transplant once he kicks it. In the manga, this is fulfilled and Nagi gets better; however, in the anime Nagi dies during Moji's battle.
Eagle Vision from Magic Knight Rayearth has overused Autozam technology (powered by mental energy), giving him anemia-like symptoms. He later reveals that he is dying from it, but he starts to recover in Cephiro in the manga.
Mikoto Saijou from Saijou no Meii was this, and then was operated on succesfully. Ever since then, he's been working hard to become trhe best child surgeon ever.
In Fate Prototype, the preliminary version of Fate/stay night, we have The Third Master. He's a bed-ridden pre-teen boy who uses his Command Spells to fully incarnate Rider in this world right before dying, and afte he kicks it Rider wants to use the Grail to revive him.
In The Twelve Kingdoms, the holy beasts named kirin choose each of the rulers for the realms. If said king or queen doesn't do well and the land suffers, they get struck with a fatal illness named shitsudou, manifesting itself via Facial Markings and physical weakness that quickly kills them. Only the full redemption (which has never happened) or the Redemption Equals Death ( Joukaku of Kei and Shishou of Sai) of the sovereign can save them from death. Kirins of both genders (Hourin, Kourin and Sairin are females; Keiki is male) have gotten it due to their masters's mistakes or madnesses: Sairin and Keiki get better, Hourin and Kourin do not.
In Gangsta the entire Twilight race is essentially compromised of Ill Boys and Ill Girls. All of them are born addicted to a Psycho Serum that they must take daily to prevent the withdrawal symptoms - only for the drug to eventually accumulate in their systems, breaking down their bodies and killing them. The whole reason they're called Twilights is because the this effect makes it a damn miracle if any of them live to hit thirty.