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 tip-toe 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. 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.
As an Ending Trope, BEWARE OF SPOILERS!!!
- Averted in Origins, a Mass Effect/Star Wars/Borderlands 2/Halo Massive Multiplayer Crossover, since the salarian STG's attempts to develop one to fight off an Alien Invasion repeatedly fail to produce anything that would be viable against the enemy though given that enemy is the Flood, this makes sense.
- Blade: Trinity: The crux of the third and final film is the use of a virus that will kill all vampires everywhere, seemingly instantaneously.
- Failed spectacularly in Serenity. The Alliance introduced a chemical agent into a colony's atmosphere, one designed to make the populace docile and well-behaved, to suppress any and all aggression (and thus resistance). The gas worked too well: most of the population became so apathetic that they just laid down and died. The others became hyper-violent berserkers, known as the Reavers.
- Zig Zagged Trope on the George Pal version of The War of the Worlds: it maintains the original ending of the Literature example below, but before the natural bacteria of Earth do the job, the military and the scientists were proposing to use bio-warfare (out of desperation, because nuking the machines didn't worked and the only option left was to try and see if it was 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!").
- A subversion in Animorphs: 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.
- The War of the Worlds. In H.G. Wells's classic novel, Earth's bacteria do in the aliens. This is kept in most adaptations, from radio to the 1950's movie. Subverted at first in the 80s TV show that just had the aliens in hibernation. Later one of the characters develops a bacteria to kill off the aliens for good.
- In Auf zwei Planeten ("On Two Planets") by Kurd Laßwitz, which was published a year earlier, 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.
- 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.
- 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 innoculate 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 diseases known to Man. A Mars cleansed of its higher life forms is then open to colonisation from the British Empire...
- 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 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.
- Nearly backfires in the New Jedi Order novels for Star Wars. 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 even 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 Robeart A Heinlein's book The Puppet Masters.
- The film adaptation has a more justified version of this, where 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.
- Stargate SG-1 had two episodes ("2010" in season 4, and "2001" in season 5) about the Aschen, an alien race 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.
- An arc in Stargate Atlantis involved an attempt to develop a virus that would turn Wraiths into humans. They never managed to make its effects permanent or figure out an effective delivery mechanism. And their first test subject, "Michael", became 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' bio-technology until the virus wears off.
- An erstwhile Alliance officer on 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.
- Used in the 7th season of Supernatural by the Leviathans against the other monsters. they used a special chemical in fast food that would make the body's of humans who ate it to be deadly to all monster species, and this is a series where everyone is a Humanitarian.
- V (1983). In the mini-series, the human resistance develop the 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.
- God has done this a few times.
- 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. And now they've found out that using them strengthens Nurgle, the Chaos god of disease...
- 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.
- In Resistance, an eleventh-hour cure is used to defeat the Chimaera and end a war that, technically, humanity had already lost years ago.
- An eleventh-hour cure is also used in Gears of War to destroy both the Locust and the Lambent.
- 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.
- 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.
- The original Sword of the Stars stated that 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. 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.
- Star Fox: Assault concluded with the heroes attacking the Aparoid queen with a electronic virus intended to induce apoptosis in their biological components. Though she is able to suppress it somehow until you finish killing her with conventional weapons.
- The eponymous [PROTOTYPE] of the Blacklight virus was designed as the version 1.0 of all future genetic-triggered viruses. Since this took place in the 1950's 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...
- 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.
- 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.
- 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.
- Family Guy: Stewie and Bertram end up in a playground war which ends when Bertram infects Stewie's side with Chicken Pox.