- The Pneumococci are pretty terrifying when they first appear, rising from the floor, their tentacles waving menacingly and their eyes glowing, as they gloat about taking over the body. And while they're easily killed by the Neutrophils, the fact that one survived and was hitchiking around the body in Red Blood Cell's cargo is Paranoia Fuel to the cells in-universe...
- Staphylococcus aureus, essentially the queen of the germs. It's bad enough for the Neutrophils to deal with a horde of bacterial invaders, but an organized horde of bacterial invaders, whose leader knows the strengths and weaknesses of the Neutrophils and knows how to fight them? Thank goodness that the Platelets were there to save the day the first time, and that the Monocytes/Macrophages could arrive in time for the second occurrence.
- The Influenza infection is portrayed as a straight-up Zombie Apocalypse, with some horrific shots of innocent cells being infected with the virus and screaming in terror as they are quickly and painfully turned...and with no way back, forcing the immune cells to kill them to avoid them infecting more.
- The scrape wound. To us, a little scratch that's no big deal. To the cells, a massive sinkhole as wide as a city block, with cells that fall through disappearing forever. To make things worse, there are also evil, alien-looking bacteria coming through the wound to wreak havoc upon the body, making the scrape wound basically a portal to hell itself spawning monsters...
- They're the good guys, but the murderous rage of the Neutrophils can be quite terrifying to behold. Even the other cells are quite shaken to see the bloodthirsty heroes gruesomely eviscerating bacteria, then eating them.
- Remember those tiny parasitic worms you see in documentaries? From the perspective of the body cells, they are gigantic kaiju-like monsters who destroy everything in sight.
- The Pseudomonas bacterium of Episode 6, who invades the bone marrow, which is depicted as a school for little children! And it's worse when he explicitly states that he has no need for oxygen and nutrients, and just enjoys tormenting and killing blastocytes (read: cell toddlers) just For the Evulz. And that's not to mention his ability to sprout extra eyes and mouths out of his body just to horrify them.
- Cancer Cell. A grotesque, twisted mutant version of a Ordinary Cell, who is a dangerous and uncontrollable psychopath who can easily masquerade as one of the Ordinary Cells. And as he mutates further, his appearance becomes even more horrific, eventually sprouting a pair of wings made of flesh!
- The main Cancer Cell looks like a normal humanoid but with mutated features. The clones he creates, however, are hideously half-formed, ranging from white-haired humanoids with empty eye sockets, to gruesome masses of living flesh with multiple twisted limbs and a single huge mouth.
- And this doesn't even go into what tumours look like, malformed faces, teeth, and misshapen limbs, all fused together and crawling on the walls and ceilings.
- Cancer Cell's ability to produce multiple additional limbs, some of which resemble Combat Tentacles made up of masses of flesh and veins!
- His backstory is even worse: Imagine being a child born with birth defects, hunted down and viciously beaten to death by a fascist police, for the sole crime of being born. Mitigated (maybe?) by the fact that said child will end the world if left alone, whether they want to or not.
- The hypovolemic shock incident. Imagine the aforementioned scratch thing, but much more lethal and life-threatening. Lots of red blood cells are lost (and dead), ordinary cells start to die due to not receiving oxygen and nutrients, making the body decay and turn into a desolate landscape, without any of the body cells being able to do anything to solve it. Thanks to a blood transfusion, things end well, but it's still the only time in the series that the body and the cells who inhabit it have been close to death, something that not even Cancer Cell has managed to do.
- The anime adaptation conveys this futility dauntingly well. After the emergency broadcast, we see everyone, from the Red Blood Cells to the Platelets, get roused for their task as they prepare to face the emergency. The next two scenes are of AE 3803, her Kohai and U-1146 wading through the wounded area, represented as a ghost town. The music has stopped. Nobody is around.
- The horror gets another layer once it's revealed that the wound which caused the hypovolemic shock get identified as a head injury. The affected area that got cut off from blood and oxygen? The human's brain.
- We also actually see the injury when some poor red blood cells have to traverse it (by chain on a tiny ledge, itself an example!), and the contrast with the scratch earlier is mind-boggling. The latter was a block-sized hole in the ground. This? This a horizon-to-horizon canyon, angry grey clouds covering the whole thing and a few intact blocks sticking out giving a terrifying sense of scale to the whole thing. It makes the platelets' attempts to fix the wound seem impossibly small.
- The heat stroke was also terrifying, due to the cells having no ability to solve it without outside assistance. The water reservoirs are all drying up, no matter how many of the back-ups the secretion cells check, the natural methods for regulating temperature are failing or not doing nearly well enough to cool the body, a germ has invaded and is gloating about taking over the whole body as the lights go out, all but Neutrophil are incapable of chasing it down and protecting the entire body, you suffocate and crowd in the ever-worsening heat, and all you really can do is strip off ever more layers of clothing, wipe the sweat off your skin, and pray... it's all a slow, gradual, but almost inevitable sort of apocalypse, and it is terrifying.
Nightmare Fuel / Cells at Work!