Daniels: They don't care; they wanna bury the town.
Salt: Ah, this is crazy.
Daniels: They want their weapon.
Salt: They're gonna kill all those people?
Daniels: Right, they want their weapon.
Salt: They're gonna sit there, and watch all those innocent people die?!
Daniels: Yes, they want their weapon!
So why choose this above Deus ex Nukina? Several reasons. While nuclear weapons are highly destructive and devastating to an enemy, a nuclear war would probably cause more problems than it solved (perpetual winter, radiation, etc.), and thanks to the Cold War, the general public is very familiar with the theoretical effects of a nuclear war. On the other hand, biological weapons are so variable that they can basically do whatever the plot requires.
So, if the heroes are experiencing their Darkest Hour, and the end looms near, this is a simple and effective way to tiptoe around a situation where Only the Author Can Save Them Now. Likewise, if you want to demonstrate how depraved your villain is, you can certainly show victims of The Plague dying in slow and horrible ways and punctuate it with mounds of burning bodies.
It can go by many names: The "Virus", The "Plague", The "Cure", The "Cleansing", etc., but it fits the same criteria:
- It targets only living things. Infrastructure and biospheres are left untouched.
- It will completely destroy the enemy ranks, or at least decimate them to the point that they are not a significant threat.
- It can be spread across the entire kingdom, continent, planet, universe, etc.
- It has a half-life long enough or communicability rapid enough that it's nigh impossible to escape.
- (Optional) It will target the enemy and only the enemy, leaving the deploying army free from consequence.
When used by the good guys at the climax, rarely do the negative affects get addressed. In Real Life, viruses are well known to mutate into nastier and unpredictable forms. A virus that once spread slowly and was cured easily could potentially mutate into a rapid pandemic. If it was bacteria/fungi, that stuff can survive outside of a living host and contaminate all sorts of things. If it was some sort of nerve agent, the issue of tainting water supplies or tainting landscape is glossed over.note And even if humans are immune from the weapon used, it usually conveniently leaves other terrestrial lifeforms and ecosystems intact. This may be justified if it's an Alien Invasion, and No Biochemical Barriers is averted.
- The crux of Blade: Trinity is the use of a virus that will kill all vampires everywhere, seemingly instantaneously.
- Outbreak: General McClintock believes that weaponized Motaba will solve enough problems in wartime to merit killing off an American town and keeping it a secret.
- The Puppet Masters: The alien slugs are vulnerable to encephalitisnote — being almost all brain, they are far more vulnerable to it than humans are. After the disease is unleashed, a military man in the area admits that he and his men are "sick as hell" but are now free.
- Failed spectacularly in Serenity. The Alliance introduced the Pax, a chemical agent of their design, into the atmosphere of the planet of Miranda, designed to make the populace docile and well-behaved, to suppress any and all aggression (and thus resistance). The Pax worked too well: most of the population of Miranda became so apathetic that they just laid down and died. A small percentage of the population had the exact opposite reaction to the Pax — they became hyper-violent and very aggressive, becoming the Reavers.
- Zig-zagged in The War of the Worlds (1953), which maintains the ending of the original story, but before the natural bacteria of Earth do the job, the military and the scientists propose to use bio-warfare (out of desperation, because nuking the machines doesn't work and the only option left is to try and see if it's possible to kill the Martians themselves). Unfortunately, the mobilization of the Martians towards L.A. means that the university the scientists are in needs to be evacuated, and the trucks that carry the equipment are stolen by desperate Angelenos who smash it all to make room (in the words of Science Hero protagonist Dr. Clayton Forrester: "they sliced their own throats!").
- Subverted: the Andalites tried to do this to prevent the Yeerks from enslaving the Hork-Bajir race (by way of a virus that only affected the Hork-Bajir), but eventually failed.
- Later, there is an attempt to use a virus to kill off the Yeerks, but because there was a possibility that it would mutate to infect their host species (such as humans) the Animorphs stopped it.
- In Auf zwei Planeten (On Two Planets) by Kurd La▀witz, which was published a year before The War of the Worlds (1898), Oß, the leader of the Antibat faction in Martian politics, wants to retaliate against the rebellion of Earth against the Martian "protectorate" by introducing the dreaded Martian disease Gragra there. However, when this morally abhorrent plan becomes known to the public, he is resoundingly defeated in the elections, enabling a peace treaty between Mars and Earth to be concluded.
- Robert Rankin trumps H. G. Wells in his Victorian Steampunk novel The Educated Ape. The War of the Worlds is referenced as having just been won, as per book, by the Martians not having thought to inoculate against Earth's diseases. But the devious Winston Churchill (in life a fervent nationalist who seriously considered the eugenic sterilization of genetically "degenerate Britons") sent some of the captured Martian spacecraft back to them, loaded with volunteer crews of terminally ill humans, with every contagious disease known to Man. A Mars cleansed of its higher life forms is then open to colonization from the British Empire...
- In Last and First Men, the millennia-long war between the Second Men and the Martians (sentient clouds of bacteria) is brought to an end using a designer virus. Unfortunately, it's too late for the Second Men at that point but they are succeeded by the Third Men.
- In Edward Willett's Marseguro, a colony of genetically engineered humans called "Selkies" is invaded by the religious fanatics who rule the rest of human space. The Selkies unleash a plague designed not to harm them and vaccinate the baseline human colonists. The invaders die but the baseline who drew them there in the first place was vaccinated and an unknowing carrier, and he made it back to Earth where it kills a large chunk of the planet. The sequel Terra Insegura covers a Selkie mission to bring the vaccine to Earth.
- This nearly backfires in New Jedi Order. The virus in question, Alpha Red, is specifically designed to target Yuuzhan Vong life-forms (and only Yuuzhan Vong life-forms). The Jedi oppose it for idealistic reasons (as not all the Yuuzhan Vong were evil, and they believed the race as a whole could likely be redeemed—and if that were not the case, they would not be a party to genocide), but the final novel (The Unifying Force) adds several pragmatic considerations to the question (which had, to some extent, been brought up before, but in that novel they go from theoretical to actual). On a planet the virus was tested on, the virus also affected a native form of life, proving the virus could mutate to infect other lifeforms. Worse, an infected ship escaped, letting the Yuuzhan Vong know about the threat... and even worse, Alpha Red was a threat to the living world Zonama Sekot, which was an offspring of the primordial homeworld of the Yuuzhan Vong. The last part of the novel includes the Yuuzhan Vong trying to crash that infected ship on the living planet, and the heroes attempting to stop it.
- This plan works at the end of The Puppet Masters. The parasitic mind-controlling slugs are vulnerable to a Venusian disease called Nine-day Fever, which is deliberately spread among them to cut back on their numbers and try to save humanity as a free race -the fever is dangerous to humans, too.
- In Jack London's story "The Unparalleled Invasion", what prevents the Yellow Peril from taking over the world is the bombardment of Chinese cities with glass tubes containing "every virulent form of infectious death," which exterminates the Chinese population in six weeks.
- In The War of the Worlds (1898), Earth's bacteria do in the aliens. This is kept in most adaptations, from radio to the 1953 movie. Subverted at first in the 1980s TV show, which just has the aliens in hibernation. Later, one of the characters develops a bacterium to kill off the aliens for good. The in-universe justification is that the Martians have evolved to the point that they no longer have immune systems and are unprepared for Earth's microorganisms.
- The 100: The Grounders send Murphy back to the 100 after infecting him with a virus that will render his people feverish and weak by the time the Grounders invade two days later. It doesn't quite work, but only through quick thinking on the 100's part.
- An erstwhile Alliance officer in Firefly made his fortune using biological weapons to depopulate communities, then he looted their untouched valuables. This was a lie, but it was certainly a plausible one.
- An arc in Stargate Atlantis involves an attempt to develop a virus that would turn Wraiths into humans. They never manage to make its effects permanent or figure out an effective delivery mechanism, but their first test subject, "Michael", becomes a major recurring villain. Never really exploited is the fact that even when it's only temporary in effect, it still renders them amnesiacs with no ability to operate their own ships' biotechnology until the virus wears off.
- Stargate SG-1: The episodes "2010" and "2001" feature an alien race named the Aschen, who conquer worlds by supposedly being nice and friendly and handing out life extending drugs... that sterilize most of the population, letting the Aschen move in and take over after nearly everyone on the target planet has died out. The Aschen also offered Earth a bacterium that could be engineered to wipe out the Gou'ld, and tried to use it on Earth when their "sterility drug" plan was uncovered.
- Star Trek: Deep Space Nine: In "Babel", the station is infected with a mysterious plague where victims develop aphasia before taking a turn for the worse. It turns out that the plague was caused by a bio-engineered virus created Bajoran resistance scientists to attack Cardassians and the computer program to replicate the virus was smuggled on board Deep Space Nine by members of the resistance. Chief O'Brien inadvertently activates the program while repairing the replicator systems and causes the virus to spread through the station. Naturally O'Brien is the first to come down with the disease and comes closest to dying. While the doctor who created the virus is long dead, his assistant is still alive and able to develop a cure in time to save the station.
- In the 7th season of Supernatural, the Leviathans plant a special chemical in fast food that makes the bodies of humans who eat it deadly to all other anthrophagic monster species.
- In V (1983), the human resistance develops a "Red Dust" that only kills the aliens, driving them off the planet. When the aliens return for the later TV series, the Dust can't be used again because long term studies have shown that any greater concentration in the atmosphere would damage the ecosystem.
- BattleTech has the Redburn Virus, the influenza strain from hell engineered by the Word of Blake. It isn't curable and has fatality rates of 30 per 100,000 and higher. Originally, it was non-contagious, but at least one variant strain has mutated. The rest of the Inner Sphere is horrified, as any biological weapons, including tear gas, are supposed to be banned.
- Warhammer 40,000: Virus bombing is one of the ways Exterminatus (destroying a planet that has succumbed to The Corruption or cannot be saved) can be carried out. As it destroys all life (and eventually, the atmosphere), its use is rather limited. Now they've found out that using them strengthens Nurgle, the Chaos god of disease...
- In Crysis 2, the Ceph are the ones trying to do this against humanity by using massive spires that release a spore that targets human DNA. Throughout most of the game, Jacob Hargreave is directing you to interface with these spires, allowing the spores to infect the Nanosuit. It turns out that the Nanosuit is capable of "reprogramming" the genetic code of the spores so that they will instead destroy Ceph DNA instead of human, and the final mission involves you jumping into a massive stream of the spores in the largest spire in New York City to wipe out all Ceph presence in NYC.
- Final Fantasy XIV: In the Stormblood story, we're introduced to the Black Rose, a chemical agent that "stops the flow of aether" in any living being's body, resulting in instant death. It was developed by the Garlean Empire for use during an invasion of Eorzea. The weapon is so potent that even several of the most prominent antagonists in the Empire are appalled by its use. The Black Rose becomes a major plot element in Shadowbringers, which reveals that in a Bad Future where the weapon was used, it went out of the Empire's control and spread across the entire world. So many people died from it that civilization and law completely collapsed, with the world being thrown back into a barbaric age of blood and tribalism. Even the Warrior of Light and his True Companions were killed by the Black Rose, killing everyone who might have even had a chance of restoring order.
- In Galactic Civilizations II: Dark Avatar, the Korath use "spore" ships that wipe out all life on a planet and leave the remainder a toxic world, which they can colonize.
- An eleventh-hour cure is used in Gears of War to destroy both the Locust and the Lambent.
- The original Halo trilogy can be loosely interpreted to end this way. The eponymous Halos are installations which can wipe out all life within a certain radius, meant to "starve" the Flood; they aren't biological weapons themselves, but they're clearly built to target certain forms of life (plants and most animals are left untouched, but anything sapient is toast). The Chief fires one at the end of Halo 3 to wipe out the Flood infecting the Ark.
- Metal Gear Solid V: The Phantom Pain has the Vocal Cord Parasites, a bioweapon developed by XOF. When the Parasites infect someone, they lie dormant until exposed to the specific soundwaves created by speaking in a specific language, after which they attack the victim's lungs and drive them into a zombie-like state of dementia as a means of spreading to new hosts. It is described as "a weapon to surpass Metal Gear", and a strain of the parasite attuned to the English language is a key component in Skull Face's plans to conquer the world by robbing the world of its most commonly-used language.
- In the Destroy ending of Mass Effect 3, the trope is inverted, with the final weapon destroying all synthetic life, including the friendly ones, and leaving the organic life alone.
- Subverted in Metroid. The Chozo created the eponymous Metroids as a biological weapon to control the rampant Parasite X on planet SR-388, which could have threatened the entire galaxy if left unchecked. Later, other races discovered the Metroid, and the creatures began to spread across the galaxy, proving to be an even worse threat than Parasite X. Then, when Samus eradicated the Metroid, Parasite X came back stronger than ever.
- [PROTOTYPE]: The Blacklight virus was designed as version 1.0 of all future genetic-triggered viruses. Since this took place in the 1950s and was headed by multiple white supremacist leaders, you can guess what this was going to be used for... except the virus gained sentience and decided that humanity itself was a problem.
- Creating biological weapons, such as viruses and such, forms the entire basis by which the events of Resident Evil occur, where various Mega-Corp factions are in a biological arms race. Naturally, this goes to show why they do not "solve" anything, as the various Bioweapon Beasts created by these companies are what's putting the world in danger in the first place.
- In Resistance, an eleventh-hour cure is used to defeat the Chimaera and end a war that, technically, humanity had already lost years ago.
- Skyfall: Following a series of unfortunate political fiascoes, conventional nukes are discredited in favor of biological warheads. This works against the protagonist, as the biological warhead that exploded smack-dab in the middle of the city has mutated any survivors who weren't in a safe room, and the protagonist's safe room is on the penthouse floor of a hotel now filled with feral mutants.
- Star Fox: Assault concluded with the heroes attacking the Aparoid queen with an electronic virus intended to induce apoptosis in their biological components. However, she is able to suppress it somehow until you finish killing her with conventional weapons.
- Sword of the Stars:
- The Liir rebelled against the Suul'ka by using a bioweapon to wipe them out. Given the species' adeptness with Synthetic Plagues everyone assumed that the bioweapon was one. This was until the sequel revealed the true nature of the Suul'ka and the "bioweapon" used to destroy them, and that seven of them survived.
- Averted in gameplay; planets hit by Biowar missiles are easily quarantined and it takes multiple warheads to wipe out a planetary population. And after first exposure a faction can research a vaccine to that specific virus.
- In Terra Invicta, the invading aliens use biological weapons to control humans and xenoform Earth. The human factions eventually develop countermeasures to both, and Humanity First and the Initiative eventually turn the tables and use biological weapons to exterminate and enslave the aliens, respectively.
- In Time Crisis 5, the Big Bad (Robert Baxter, previously the protagonist of Time Crisis II) steals a drug three years ago that turns its victims into mindless zombies, or as his former comrade Keith Martin puts it, makes them incapable of feeling pain or fear. Robert then reveals that he has a bomb that can be detonated to spread the drug to entire cities, with New York as his first target.
- Genocide Man takes place after several extremist groups used open-source biotechnology to kill billions. The titular Genocide Project is an international law enforcement agency that uses targeted plagues to wipe out "genetic deviants" and their creators. It later turns out that the Genocide Project unleashed a gene therapy virus intended to make humanity immune to all the open-source designer plagues out there (but not their own). It killed a billion people.
- Codename: Kids Next Door: In "Operation: V.I.R.U.S.", it's revealed that chickenpox was invented by teenagers in order to torment kids, so the Kids Next Door retaliates by inventing an acne virus and unleashing it on Cree's prom.
- Family Guy: Stewie and Bertram end up in a playground war which ends when Bertram infects Stewie's side with chickenpox.