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Specifically, if deployed on a town. The buildings, animals (flora and fauna) would be completely untouched, but not a single human would survive the blow.

Secondly, if this is possible, what gene, DNA, and/or disease, would the weapon target in order to specifically target humans?

If it's not possible to wipe out humans without affecting the ecosystem, what gene could be targeted to do the maximum damage to humans and minimal damage to surrounding environment? How would environment be impacted?

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    $\begingroup$ Pretty sure this is a dupe, but I don't have that privilege level yet. The easiest way, as done in several SciFi stories as well as "Children of Men," is to render everyone sterile and wait 100 years or so. $\endgroup$ – Carl Witthoft Jun 24 at 19:16
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    $\begingroup$ The Black Death killed one half of the population of Europe (and more than one fifth of the entire human population of the world) in the 14th century. Many villages were completely deserted -- Wikipedia says that only in England 1300 villages were completely abandoned. Other than killing lots of humans and some rats the plague had no effect on the environment. $\endgroup$ – AlexP Jun 24 at 19:30
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    $\begingroup$ Welcome to the site, Cherry. Please note that we strongly encourage users to wait at least 24 hours before accepting an answer. We have users all over the world; accepting early (especially within an hour of asking the question) may discourage others from posting potentially better answers. $\endgroup$ – Frostfyre Jun 24 at 20:24
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    $\begingroup$ Welcome to Worldbuilding! We're glad you could join us! When you have a moment, please click here to learn more about our culture. (a) given enough time and money, almost anything is possible. Some are already speculating this ability. And considering a virus already does what you want, what are you asking? (b) Hopefully you're not asking for details as this site is probably already on the U.S. Homeland watch list. $\endgroup$ – JBH Jun 24 at 23:16
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    $\begingroup$ Keeping animals and plants alive is the hard part. Neutron bombs was created specifically to kill living things (people, animals, plant..) but not damage non-living objects. The radiation is also short-lived allowing the attackers to quickly go in and take over the town after the bombing $\endgroup$ – slebetman Jun 26 at 8:09
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Prion diseases, specifically Transmissible Spongiform Encephalitis (TSE).

Prions are a protein that, as near as we can tell, all mammals produce. These proteins are not broken down during digestion, so contaminated food is a popular vector.

Creatures typically create their own variants of the Prion Protein (PrP), so Prion diseases are typically not zoonotic (they do not cross species barriers). However, cattle do have a zoonotic form with humans, in Bovine Spongiform Encephalitis (BSE, Mad Cow Disease).

It takes humans nearly 20 years to be symptomatic, but once symptoms begin, progression is rapid (months) and always fatal. Prion diseases are neurodegenerative disorders, which will cause behavioral changes, rapid cognitive decline, and intense pain.

Prion diseases work by an animal taking in a malstructured PrP that, through some mechanism that isn't definitively identified yet, slowly changes normal PrPs to the malstructured form. Since normal PrPs are necessary for these creatures to live, the immune system does not respond to the malformed PrP.

The most common form of transmission of Prion diseases is cannibalism. Since humans don't generally eat each other, this is incredibly rare. However, improperly buried corpses can contaminate water supplies and crops. Additionally, PrPs are very resilient in the open air -- An intentional delivery system would be to just spray it directly on crops.

The malformed PrPs can appear spontaneously in animals, so people don't need to be directly exposed in order to get Prion diseases, but spontaneous infection is rare, and there is no known mechanism to intentionally create a spontaneously malformed PrP. Plants have been known to take up animal PrPs, so you can't simply wash your fruits and vegetables, as the infectious part is already inside your food.

The only plausible (though unlikely) way to infect the entire world would be orbital chemical weapons caches that accidentally deorbit, spreading large quantities of malformed PrPs throughout the entire atmosphere and eventually depositing them on every field -- or some other similar global contamination mechanism. It would take a decade before the first people become symptomatic.

Animal-based vectors are out... i.e., you wouldn't be able to weaponize mosquitos to carry it from person to person the way malaria is carried, because humans exist in all biomes. The only other animal that shares all biomes with humans are rats, and since humans no longer have fleas like we did back in the days when rats carried the plague throughout europe, a weaponized Prion disease wouldn't be transmissible that way, either.

Global TSE infection would have the most gentle impact on the environment of any human die off event. There won't be planes crashing, etc., as symptoms appear slowly and are not immediately debilitating. Power plants and dangerous reactors will be shut down in controlled manners. Residual pollution will slowly leak into water supplies as storage containers break down, but this is a decades-long process rather than an immediate event.

Pets and farm animals will be released and hunted by natural predators in a more drawn out manner... Pigs will be the most successful invasive species.

As far as the die off itself... Some people will start getting more violent as the pain and cognitive decline drives them insane. There won't be roving bands of violent half rotten zombies; they'll just take their frustration out on the people closest to them, suddenly start getting very weak (their brains won't be sending strong signals to their muscles), then over the course of a couple days, lay in bed (or in an alley, or wherever) and eventually stop breathing. The die off will take a very long time in human timescales, but all humans will be dead within 25 years of releasing the weaponized PrP into the upper atmosphere.

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  • $\begingroup$ Prion diseases take decades to even manifest and then up to another decade to kill the host and we still don't understand how or why they occur or exactly how they operate so the idea of weaponising a prion disease is a very large stretch. $\endgroup$ – Ash Jul 19 at 13:58
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Viruses are highly species-specific, i.e. a human virus will only kill humans. So a biological weapon will do the job. The best virus to use is beyond my knowledge, and I am a bit concerned a terrorist would read this. If we stick to sci-fi convention, then zombie virus is the most obvious choice. But realistically, even the deadliest viruses leave some survivors who happen to be immune.

It is not possible to "to wipe out humans without affecting ecosystem". We are part of the ecosystem. Accidents at factories and power plants will generate a ton of immediate environmental damage. Nuclear station meltdowns, oil spills, fires, chemical factories exploding, etc. Dam breaks will wipe everything downstream. Rain will wash trash and debris into rivers and oceans.

Farming industry will have longer effects: lots of animals will die, creating temporary feeding ground for scavengers, and then starvation and death once no more animals are available. Other animals will escape, probably briefly consuming all plants in immediate area.

Our pets (as long as they can get out) will be an interesting addition to wildlife.

But in 10-20 years, nature will recover and take over. Are you willing to wait that long?

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    $\begingroup$ If the weapon only killed humans, there's no reason for the animals to mutate to "survive"; they weren't even affected. $\endgroup$ – Nelson Jun 25 at 4:55
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    $\begingroup$ Correction: Viruses are mostly species specific. Rabies being perhaps the most famous counterexample. $\endgroup$ – Arthur Jun 25 at 8:45
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    $\begingroup$ @Cherry The problem is that without humans around, no virus, bacteria or parasite will survive to be dangerous over 150 years. You can work around this, though - the virus might infect e.g. rats, without actually doing them much harm. The same virus can easily kill a human while surviving and reproducing in other animals without serious harm (see: essentially every virus that crossed the species boundary ever; compare a cold to something like the flu). $\endgroup$ – Luaan Jun 25 at 10:01
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    $\begingroup$ Pet tarantulas will face perhaps the worst fate, slowly starving in tiny plexiglass cubes. Hamsters will be only slightly better off, taking less time to starve than the spiders. $\endgroup$ – John Dvorak Jun 25 at 10:06
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    $\begingroup$ A deadly contagious human-specific virus is pretty much exactly what smallpox was before we eradicated it. If someone revived it, modified it to make it even more virulent and spread through the air... $\endgroup$ – Lee Daniel Crocker Jun 25 at 20:09
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Targeted robotic micro cluster munitions.

Some humans might be resistant to a biological weapon, or be able to be isolated before infection sets in. Few humans will be resistant to a micro shaped charge to the head...

Designed as hoppers, such devices could be sized such that they wouldn't leave notable damage to any part of the surrounding environment.

For slightly more alignment and targeting effort they could employ a contained "bolt gun", and each munition could be reusable.

If employed using a non- learning AI, then there is effectively zero target drift as could be found in a biological weapon.

After initial deployment the munitions could remain in standby mode to catch any straggling basement dwellers, or remotely disabled to allow the region to be occupied.

They could also be programmed to only activate on "safe" targets to avoid collateral damage. No killing people driving cars on the highway or anyone at home with a stove on kind of thing.

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  • $\begingroup$ I assume the robots would be macro-sized, and humans won't have adequate protection or weapons to fight them off? $\endgroup$ – Alexander Jun 25 at 16:27
  • $\begingroup$ @Alexander doable with fist sized or smaller munitions. Humans wouldn't have any more reliable protections against such devices than they would for known bio-weapons. Assuming the weapon is even known about prior to deployment... Odds of survivors being left and able to create adequate counter measures is 'highly limited'. $\endgroup$ – TheLuckless Jun 26 at 17:05
  • $\begingroup$ so you are essentially talking about an army of "Terminators", right? Unless this army is vastly superior to humans' one, I foresee a devastating war with possible use of nuclear weapons. $\endgroup$ – Alexander Jun 26 at 18:12
  • $\begingroup$ No, I'm talking about carpet bombing a region with 3-5 times the local population worth of "smart grenades/mines" that can use audio/visual sensor input to detect and launch themselves at human targets while safely ignoring other things. $\endgroup$ – TheLuckless Jun 26 at 18:38
  • $\begingroup$ Well, somebody or something has to deliver the bombs. $\endgroup$ – Alexander Jun 26 at 18:41
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For the question as stated, it is not necessary to use a bioweapon; Slaughterbots programmed to consider all humans "the bad guys" would do, too.

In case you cannot see the video:
The concept is a swarm of AI-controlled drones, each equipped with a shaped charge designed to penetrate a human skull.
The drones are more maneuverable than a human hand, so once the recognize a human skull, they fly near it and explode the charge.
Are these slaughterbots realistic? Sort of:

  • Physical countermeasures are possible - just deny them entry using nets.
  • Counter-countermeasures are possible, too: shaped charges can be designed to cut nets.
  • It is pretty realistic that drones can evade hands, baseball bats, or other attempts to shoot them down. They could be shot, but they are a small moving target, so a drone on attack course would be extremely difficult to stop.
  • The payload (shaped charge) is scarily realistic: after all, you need the equivalent of just a single pistol shot. The shaped charge is just a way to build a pistol shot that does not need a pistol with trigger, barrel, or grip, which is easy if the shooting platform is a fire-once-and-forget thing anyway. This is the core element that makes the video so scary: You can kill anybody with just 20 grams of explosive (I bet you can reduce that further).
  • Current-day AI is not that smart. Expect drones to attack things that only look vaguely like a human head, and occasinally attack something random that just happens to trigger the "right" paths in their neural network programming. A Slaughterbot-infested area would still be too dangerous to enter.
  • For the question as stated, any Slaughterbots deployed in the area will eventually run out of energy and become inert. If a some power source is available, the danger can persist until the power dies.
  • The bots could be programmed to deposit their charge and a trigger when the battery runs low. This would pepper the area with anti-personnel mines, which can prolong the thread until the explosive degrades; this will take years, decades, or possibly longer.
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    $\begingroup$ not everyone knows what is a slaughterbot. please add the relevant part of the linked page to your answer, so that even if the link is broken (it happens more often than not) the information will still be available. $\endgroup$ – L.Dutch - Reinstate Monica Jun 25 at 8:14
  • $\begingroup$ I think your answer is good, and imho should be the correct answer. However, around here we prefer answers to be a little bit longer than a single sentence. Maybe you could flesh out a bit (and keep in mind that lnks, such as the oe to the vid) can go away over time. Oh, and welcome to worldbuilding! $\endgroup$ – Burki Jun 25 at 8:35
  • $\begingroup$ Screamers!!!!!! $\endgroup$ – slebetman Jun 26 at 8:16
  • $\begingroup$ I added the requested detail. I didn't have the time for that when I first wrote the answer, but now I do :-) $\endgroup$ – toolforger Jun 26 at 10:13
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Face recognition drones

You can already find some pretty crazy simulations of swarms of bird sized drones, air dropped, that will just run to your head and detonate a directional explosive charge. add some bigger drones for door or window breaking, and you get a perfectly plausible mass killing weapon. Crazy expensive, but with minimal structure and nature damage. You can even set it up to recognize facial patterns to target specific range of people (the ethical one would be to not kill children, the unethical that come to mind is focusing a special ethnicity or group of people), or even specific people, like politic opponents, registered criminals, or people still using facebook. Any database with faces will do.

the limits of the weapon will be bunkers or any better defended place, but you can still adapt with big mama drones and stuff like that. firearms will do very little against your drones if they have correct flying patterns. A helmet won't protect you from the explosive charge from the drone (a slug ammo will go right throught a helmet) but it can protect your face to be recognized, so it comes to a recognition warfare (full humanoid body detection counter the helmet), to defend against this recognition you'll try to get out of the pattern, etc.. Also some countering will happen with jamming comms between drones and the "motherships" that coordinate the attack, but it won't stop much.

All in all, you should evaluate the kill cost to 100 dollars per head (single use drones are quite cheap), maybe 2000 dollars for a door opening, adding maybe 1M$ for highly amoral software dev and you will also need some delivery system. But then you can kill 95% of a city population in a night.

That's f*cked up, by the way.

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  • $\begingroup$ Rifles won't be effective against drones, but shotguns will be. Using crazy flying patterns may help, but then drones will quickly run out of "juice". $\endgroup$ – Alexander Jun 26 at 18:23
  • $\begingroup$ @Alexander shotguns will be more effective than guns, but can still be overrun by multiple drones, even in closed spaces. a simple "spread out" flying formation (which is essential anyway in swarms) will need you to cover an entire door and to shoot every second or so if there is enough drones. The best defense will probably be hiding, or enough walls and nets to stop a randomized search and destroy pattern. $\endgroup$ – ncalep Jul 1 at 7:31
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    $\begingroup$ sure, with sufficient numeric superiority any defenses can be overrun. $\endgroup$ – Alexander Jul 1 at 16:02
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This is likely going to be a disease

  1. Human-only disease. A highly virulent and highly lethal disease which affects only humans can devastate the entire world before humans can do anything to react to it. Incubation period should be long enough that normal quarantine methods are not going to be very effective. The downside here is that there inevitably be a large number of survivors that would isolate themselves from the rest of the world, and once everyone affected would die, the survivors will just start over.

  2. A disease that is fatal to humans only. Imagine a disease that infects all mammals, but is fatal to humans only. Without having a cure, a world in which every animal presents mortal danger would be very hard and perhaps not survivable for humans. If disease's Natural reservoir is very large, it just can't be eliminated, and pockets of human survivors would be doomed as well.

  3. Nanobots. Today, the world "nanobots" is very generic and almost synonymous with "magic". Anything that is scientifically doable, can be performed by nanobots. So imagine zillions of nanobots infecting all living things on Earth, checking their DNA and mercilessly killing them if it has human composition.

All mass destruction methods that don't have DNA tuning are not going to be selective enough to avoid large collateral damage among animals. And I also assume that dying off of the farm animals and pets is acceptable.

P.S. Note about the tech level. Options #1 and #2 can be implemented tomorrow, if researchers stumble on the right pathogen. Medical science would unlikely be able to counter it in a short to medium term, unless it makes a similarly lucky stumble on a vaccine. Option #3 requires advanced tech. In that scenario, it may be likely that humanity would have a readily available technology to counter the threat from nanobots.

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  • $\begingroup$ Scenario three could be stopped using modern day technology. EMPs, and electro magnets would easily wipe out nanobots. $\endgroup$ – Garret Gang Jun 25 at 18:41
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    $\begingroup$ @Garret Gang you are making assumptions about nanobots' technology and vulnerabilities :) $\endgroup$ – Alexander Jun 25 at 18:49
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    $\begingroup$ @Garret Gang EMP works because target's electric circuitry behaves like a radio antenna. Nanobots (in a traditional electronics-based sense) would make poor radio antennas because of their small size. I'm not saying they will be invulnerable to EMP, just the EMP would need to be very powerful to affect nanobots. $\endgroup$ – Alexander Jul 1 at 19:52
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Technically no, a weapon of mass destrcution (WMD) has to devastate the scenery and everything in it, a mass casualty weapon that only kills humans is pretty easy though. This is going to be a biological weapon, Smallpox is a good candidate as are Ebola and Marburg, they may infect other animals but are only really lethal to humans.

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  • $\begingroup$ Smallpox won’t get 100%. I don’t know how many are naturally immune, but I know there are at least two of us. $\endgroup$ – WGroleau Jun 25 at 7:11
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    $\begingroup$ @WGroleau It's extremely difficult for anything to kill off 100% of a population. Fortunately, it isn't necessary - there's very few populations of humans that could survive mass dieouts even if they themselves are immune. People depend on other people. Granted, even then, you could imagine some small pockets of humans to survive for a hundred years; it depends on how thorough the OP needs to be. $\endgroup$ – Luaan Jun 25 at 10:06
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Unless the bioweapon was released simultaneously, I don't think it would be effective in wiping out the humans. If a city was targetted, humans would study the effects and look for ways to survive and counter the weapon.

A biological weapon that targeted the human reproductive cycle might be more effective. If it was highly contagious and initially had no side effects, or very few, but prevented the pregnancy from coming to term, might infect a large percentage of the population before anyone was aware of it. If it scared the uterus of an infected woman that became pregnant, then they would not be able to get pregnant later, if the humans worked out a cure. In a generation or two, you could wipe the humans from the planet.

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Depending on how scifious you're willing to get, another possibility is a swarm of highly reproductive, deadly venomous locust-like creatures, genetically engineered to only be attracted to, and enraged by human pheromones - perhaps humans are all they can eat too, so they will die off soon after their job is complete. (Unless they evolve to eat other creatures too)

Of course the problem is containing them. But if, say, they can't cross the sea, you could use them against an island.

If you wanted more control, they could be little A.I. flying robots instead of biological insects...

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