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Trying to figure out a post-apocalyptic scenario and looking to stay somewhat within the realm of possibility. Given how tech levels MIGHT be in a 50 years, what would you say would be the most likely cause of a catastrophe that wipes out maybe 90% of Earth's population?

I know a giant asteroid is out of the question - IIRC we've pretty much mapped out every asteroid that has a chance of coming close to the Earth and would have a lot of advanced warning. A comet, maybe? They tend to be more unpredictable.

Nuclear war also seems unlikely as it would essentially mean mutually-assured destruction and I can't see why major nations would want themselves to be annihilated as well.

How likely is a bio-weapon? Would a super-contagious viral plague work too fast and be quarantined too easily?

Or would the catastrophe be slower and more insidious, brought on by climate change and dwindling resources, leading to reduced territory and infighting/civil wars around the world?

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    $\begingroup$ Close voter: this kind of question can't avoid being opinion based, and yet it's a very reasonable worldbuilding concern. $\endgroup$ – Zeiss Ikon Oct 8 at 17:40
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    $\begingroup$ Nuclear war, in today's world, is more likely to be initiated by a minor nation, one whose leadership either has lost touch with reality, or who has come to believe they have nothing to lose (say, one with a three-generation hereditary dictatorship). $\endgroup$ – Zeiss Ikon Oct 8 at 17:43
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    $\begingroup$ @ZeissIkon: It is not a reasonable question. A resonable question would be: "I have the following scenario for a worldwide unweiling. A does B, and this leads to C, with D reacting so and so. Is this realistic?" As the question stands, the querent is asking the community to develop a starting point for their world. $\endgroup$ – AlexP Oct 8 at 19:24
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    $\begingroup$ @Shadowzee OP is asking for "most realistic" -- that's a single best answer, and if it's a list question, it's a short, bounded list. These qualities are generally held as on topic across a wide range of SE sites. $\endgroup$ – Zeiss Ikon Oct 9 at 11:05
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    $\begingroup$ Asteroid would be the most likely option. Spotting it does not guarantee the ability to stop it, and many large ones do fly by without being spotted. A nuclear exchange between the US and North Korea/Iran or India/Pakistan would not spread (other countries are not suicidal), and even a nuclear war between the major powers would not kill as many people as predicted, as many nukes would be directed at enemy silos, not cities, as arsenals are smaller than during the cold war, and as nuclear winter is probably a myth. $\endgroup$ – Richard Smith Oct 9 at 14:00

14 Answers 14

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The Four Horsemen

Global warming is making crop failures look likely. Famine can kill large numbers of people and historically has done so often. Tens of millions died in the midcentury famines in China and the USSR. If global warming makes the tropics unsuitable for agriculture that would lead to famine in Africa and India for sure and possibly China. Starving people do not drop in their tracks - these people will migrate and destabilize the regions where they show up.

Migration of desperate people leads to conflict. This is happening now. https://www.nationalgeographic.com/news/2015/3/150302-syria-war-climate-change-drought/

If you add war (and its friend pestilence) to this context you might reach your 90%. Major nations with a view for the long term do not want to be annihilated but in the context of famine and desperate chaos, people who come into the possession of nations might not have the same long term view as people do now.

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    $\begingroup$ To be fair, the mid-century famines of China and the USSR were because of Communist government policies, not the environment. Otherwise, your arguments about how people deal with famine, and the repercussions hold up. $\endgroup$ – Stephan Oct 8 at 18:25
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    $\begingroup$ To be fair, this is the opposite of what we have seen so far since carbon dioxide is plant food. This NASA article is summarizes an article in Nature. $\endgroup$ – Gary Walker Oct 9 at 12:17
  • $\begingroup$ @Stephan Are you saying capitalist policies were that much better? $\endgroup$ – Alice Oct 9 at 13:18
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    $\begingroup$ @GaryWalker: Really? I mean, really? Even your own source doesn't support your statement... Read to the end of the article, where it states that just a mere 50% of human CO2 emissions get caught by photosynthesis, that plants will saturate, and that the remaining CO2 is still having significant impact. (I've so had it with this armchair denial...) $\endgroup$ – DevSolar Oct 9 at 13:30
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    $\begingroup$ @IronEagle Quite a different subject, and one I'd be willing to discuss if not for a) it having been discussed to death (no, it's not good for vegetation either if the trend continues), and b) this ain't a discussion forum. $\endgroup$ – DevSolar Oct 9 at 15:53
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The most likely suspects

  1. Molecular Nanotechnology (either weaponized or accidental). ~31%
  2. Artificial Super Intelligence (terminator scenario). ~27%
  3. Large scale wars, including nuclear and terrorism. ~27%
  4. Engineered or natural pandemic. ~15%

I've summarized the Source and redistributed the probability under the assumption that there was such an event. Note that the source references extinction by 2100 not 90% by 2119, however, I think that you would need an extinction level threat to kill 90% of people, it just needs to be slightly milder, and none of the above scenarios are likely to have an exclusive 100% kill condition.

Notes on other answers:

Climate change is likely to have an impact on the world moving forward, but it's extreme to say that it could kill 90% of people in the next 100 years. Millions possibly, billions probably not, over 6 billion is extremely unlikely.

Famine as a global risk is unlikely due to how diverse food sources are, a long term problem might get you as high as 50% but as people die the demand for food decreases, and people move to the places where they can get food (or else die) so the problem quickly remedies itself.

Alternatively there's always the rapture

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I also believe climate change is the most likely source of catastrophe over the next century. Not only starvation and migration-induced wars, but weather pattern changes that could lead to much more rapid changes in habitability of populated regions. For instance, a five year drought in Brazil would probably depopulate half of South America, and have knock-on effects I can't readily foresee.

There is still the risk of nuclear war, as well. Major nations may not start it (as you say, MAD makes this MADness), but a small, impoverished nation with a hereditary dictatorship and a new nuclear arsenal might well start a fight they think their enemy hasn't the will to pursue -- and the counterstrike might trigger a secret treaty (like the one that put the US into WWII in Europe as well as the Pacific), leading to a less restrained exchange.

I still wouldn't rule out an asteroid, either -- at least as the trigger. If a rock like the one that exploded over Chelyabinsk came in on a more vertical trajectory over a major city, it might be days or longer before the victim city's national government knew it wasn't a nuclear attack -- and by then it would be days too late for whoever they thought was likely to strike that way, that week. And again, counterstrikes would escalate.

At least a nuclear war would offset a lot of global warming...

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    $\begingroup$ Note: A nuclear winter is followed by a nuclear summer... so in the long run, nuclear war would still exasperate global warming $\endgroup$ – Nosajimiki - Reinstate Monica Oct 8 at 18:10
  • $\begingroup$ Amartya Sen said "there is no lack of food; there is a lack of money to buy food." A drought in Brazil would probably not depopulate half of South America because the non-agrarian economy would still produce enough income to buy basic food, together with aid and migration. $\endgroup$ – Peter - Reinstate Monica Oct 9 at 6:53
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    $\begingroup$ If the Chelyabinsk meteor you said in your post happened in the early eighties, during events like Able Archer, it would be very difficult to convince the soviet leadership not to strike back in the hours after the event. Remember that Mayak and its nuclear facilities are near Chelyabinsk. $\endgroup$ – Geronimo Oct 9 at 13:45
  • $\begingroup$ @Geronimo I don't know Soviet/Russian geography well, but the same is true if such a thing had struck, say, Hanford, Washington (nuclear fuel and bomb metal production facility, waste disposal, etc.). Especially if there wasn't the long near-horizontal fireball stage due to near-vertical entry angle. $\endgroup$ – Zeiss Ikon Oct 9 at 13:52
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    $\begingroup$ @Geronimo - I disagree. Chelyabinsk is well known largely because it was recorded. Widespread recording would not have happened in the '80s. The meteor also wouldn't have triggered their early warning systems. In the absence of that, the first they would have known of it would have been reports coming in from the ground giving details of the damage. It would have been immediately obvious that it wasn't a nuclear strike. It's quite likely that an event like Chelyabinsk could have occurred in USSR and simply never become public knowledge outside of the local area and the Kremlin. $\endgroup$ – Spudley Oct 9 at 14:06
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The biggest problem with coming up with a disaster scenario here is the need to kill 90% of the population. That's going to be a very difficult thing to do. Most of the disaster scenarios mentioned in other answers (climate change, a Carrington event, a limited nuclear war) would not come close to that.

The IPCC says climate change could cause a loss of global GDP of .5-2% by 2100. That's bad, but it's a far cry from killing 90% of the population. Even the most extreme scenarios for climate change would not kill a large percentage of the population.

A strong Carrington-level event would certainly kill a lot of people. Even a short-term (months) disruption of power grids and communication networks and electronics in general would result in large losses of population as food distribution and production failed, but 90% is not realistic.

A nuclear war - even a large-scale one - would not kill 90% of the population. Even at the height of the cold war, with potentially thousands or tens of thousands of warheads detonated in a conflict, no one was predicting anything close to 90% loss of life. In 1979 the Office of Technology Assessment issued a report to Congress in which they estimated loss of life in a full scale exchange between the Soviet Union and the U.S.A as being 35-77% of people in America, and 20-40% in the Soviet Union.

The Effects of Nuclear War

To be fair, they admit that there are a lot of unknowns and some of their estimates are just educated guesses. Still, even if you accept the worst case, it's hard to get to 90% because there are a lot of countries that are very spread out and likely not to be targeted. Australia, Indonesia, Africa, South America... Lots of population would likely survive, and many of them would be in the Southern Hemisphere where even fallout patterns can't get to them.

There are two disasters that could easily kill 90%, or even wipe out humanity completely. Those are a giant impactor (asteroid or comet), or a biological weapon. We discover new comets and asteroids all the time, and while we have no doubt found all the close ones that could threaten the Earth with an extinction-level event, you can always have one appear from outside the solar system. We'd know it was coming for a long time, since anything big enough to destroy mankind would be spotted long before it got here, but we'd be utterly unable to do anything about it.

A biological weapon could wipe out humanity if it was designed to do so. Imagine a virus that has no symptoms other than to make the person who contracts it be sterile. Or a virus that is contagious for a long time before it begins to show symptoms, and then you die shortly thereafter. There would be no way to find the carriers, or even to know the disease existed before it had spread through enough populations that it could never be stopped. Maybe 10% of the people have an immunity.

If you had said that 1/3 or even 1/2 of the people die then I think you could plausibly use any of the disaster scenarios mentioned. But 90% is a really hard number to get to. I'd go with either a major impact or a biological weapon.

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I would argue famine is a good contender in general.

Over the last 100 years, we've come to rely on refrigeration and a global network of food shipment to supply local needs across the globe. As a planet, it's almost as if we've collectively decide to put all the production of any particular farm production into one ecosystem, and just ship the products everywhere it's needed.

This effectively means that we've concentrated the population of every species of domesticated crop and animal into regions that could easily be swept with disease and exterminated entirely.

I read recently that the African Swine Flu recently destroyed 1/3 of the world's pork production with very little fanfare. Apparently with this much concentration it doesn't take much to cause big problems in our food supply.

This route is also a good one to pursue if you want to avoid modern politics bleeding into your world. No one country and no one political party has given this particular issue a stigma, which lowers the risk of your audience getting offended by perceiving your world as a soap-box for your own ideologies.

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another Carrington event

A solar based EMP would essentially throw us back 150 years in technology.

No major city would survive the first winter. Only hardened electronics, or those stored in nice faraday cages would survive. An estimate of 1/2 to 2/3rds of the worlds population would be dead within a year.

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  • $\begingroup$ How does this add up to 90% of the population gone? $\endgroup$ – Zeiss Ikon Oct 8 at 18:58
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    $\begingroup$ This could be nice as a starter for further conflict / famine / drought/ natural catastrophes etc, but I agree with @ZeissIkon that alone it probably wouldn't suffice. $\endgroup$ – Whitehot Oct 9 at 9:50
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    $\begingroup$ @ZeissIkon follow the progression... Powergrid and all modern tech GONE. Fires everywhere, like here were during the carrington event, hundreds of millions without power, entire cities burning, no resources to fight the fires, mass deaths, disease from the corpses, et cet... follow the dominoes $\endgroup$ – Richard U Oct 9 at 12:51
  • $\begingroup$ @RichardU I was referring to the answer (presumably based on its reference link) saying 1/2 to 2/3 -- which is not the question's 90% request at all. Sure, it could get worse -- but apparently the folks who estimate this stuff don't think it would. $\endgroup$ – Zeiss Ikon Oct 9 at 13:05
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    $\begingroup$ @RichardU Maybe not directly a disease from the dead bodies, but in such an event cannibalism could lead to wide transmission of prion diseases? $\endgroup$ – IronEagle Oct 9 at 15:54
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1) Do not throw out asteroids - they are not all mapped! And they are not tracked 365/24/7. Asteroids have a tendency to change their course due to the influence of the planets (mostly Jupiter). And even quite large asteroids (up to 1 km in size) will be detected only months before impact. That is the reason why asteroids that would pass by millions of km from Earth are considered as threatening.

2) Do not throw out mass nuclear bombardment (it is the most probable event). It may not be war, but software glitch, humanity-hating fanatic conspiracy or smth like that. Even 1% of the total nuclear arsenal (like say, Indo-Pakistan "local" nuclear war) is enough to cause near-extinction of humans.

3) Mass volcanic eruption - would have the same effect as mass nuclear bombardment. If it is not detected years before. If humanity is prepared (shelters and supplies), it could be reduced, but not that much. But if not - it would be truly catastrophic.

There will be no realistic extinction for:

  • Global warming, while it would destroy some (many) countries and species, it would make Earth more habitable by humans and cows. Humanity would benefit from it.

  • Pathogens and bio-weapons, while being deadly, they have negative feedback: the more they kill, the slower they spread. Even without organized counteraction it would not depopulate even one region (without help of invaders - see America's history). The most deadly infection killed hundreds millions of people, but it was only about 5% of all Earth population. To kill 90% - the situation should be 19 times more desperate, than it was during The Great War.

  • Non-nuclear war of any kind. It has the same "issues" in killing humans as pathogens have: the more it kills, the less resources left, the less intensive war is. Given (by experience), that no more than about 10% of population can be at war and no more than about 30% can work for war (producing war supplies) and real-life weapon efficiency is quite small (like only 1 of 3000 of all bullets kills someone) global non-nuclear war would not kill more than about 50% of all population.

  • Some "evil technology", like nanobots or black hole generator, while it can be deadly, they are not true "near future". We even do not sure if it can be possible or impossible. They all are pure speculations. We can "imbue" them with near magical properties, but this would not be since, but fiction.

90% of current population is a lot of people. While many things can kill billions - it is not enough to accomplish it.

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Impact/bombardment from the Taurid Meteor Stream

Example; https://www.youtube.com/watch?v=N9So2SfJzE8

Every year we pass through/by this meteor stream twice, which originated from the breakup of a much larger comet named Encke (we've actually been hit with things from this stream a few times, https://www.space.com/beta-taurid-meteor-shower-tunguska-explosion-planetary-defense.html ), and it's believed an impact about 12-13,000 years ago resulted in the wiping-out of all the largest mammals from the northern hemisphere (woolly mammoths / woolly rhinos etc).

It's entirely possible we will be hit by more of these in the future, and entirely possible that we will have allowed it to happen by focusing on 'terrestrial politics' rather than developing absolutely possible to create solutions to the problem, so there's a bit of "the hubris of man and his ego" angle to the story too.

This could create enough damage to basically set civilisation back to 0 without scraping all life off the planet, or it could damage society just enough that supply lines are irreparably damaged and people have to revert to a sort of neo-hunter-gatherer state within the corpse of ruined cities. There's a lot of room for play depending on the amount of damage you're willing to do in the apocalypse, because the Taurid Stream is such a large number of objects nobody could ever turn around and be all "well ackchually a single object of x y and z dimensions wieighing bla bla bla would/wouldn't do what you said it did", so there's that too.

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Smallpox

The World Health Organization (WHO) describes smallpox as:

[O]ne of the most devastating diseases known to humanity. Smallpox is transmitted from person to person by infected aerosols and air droplets spread in face-to-face contact with an infected person. The disease can also be transmitted by contaminated clothes and bedding... Two forms of the disease are recognized, variola minor with a mortality rate of approximately 1%, and the more common variola major with a mortality rate of 30%.

So basically, it's a highly infectious airborne virus that kills a third of the people it infects - pretty scary stuff.

Smallpox has been eradicated - it's one of the greatest successes in modern medicine. But some samples remain for research and some emerging technologies enable scientists to create new poxes from scratch.

Smallpox and other Old World illnesses are believed to have killed approximately 90% of Native Americans, partially because they had no communal immunity to these diseases, and partially because their societies collapsed in the aftermath of these mass deaths.

So if someone were to - deliberately or not - re-introduce smallpox into a world without any real immunity, it could potentially crash the entire global economy, leading to massive famines and wars.

How do you feed your population, and also quarantine it? How do you stop the flow of infected refugees? How do you react when a neighboring country takes actions you think increase your country's risks?

As people panic, and demand action from their governments, the risk of taking actions that make it worse are high. I could see this spiraling out of control pretty easily.

All because someone re-created or release smallpox - whether on purpose or not.

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Biological warfare/terrorism aimed at hitting food production could contribute.

Say a pathogen aimed at disrupting grain supplies is unleashed by a malicious state or terrorist organisation, and proves more effective & resilient than intended.

  • Terminator gene fails (the pathogen keeps spreading forever)
  • The pathogen hits more forms of grain than intended (eg. hits rice, corn, oats and more when intended for wheat alone)
  • It spreads further than intended due to a combination of retaliatory action and incompetence.
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My personal opinion is that genetic engineering will cause an ecological catastrophe of one kind or another.

Many times a non-native species was introduced in a habitat it caused havoc there. Australia is a prime example with rabbits and cats. The genes on earth form an "ecology" in themselves, divided in "habitats". While there is a lot of interaction within and between species, there are also natural barriers which normally prevent spreading certain genes. Mad cow disease is an example of what happens if you break such natural barriers, like feeding animals to herbivores; even if it didn't involve genes, just proteins. Genetic engineering is ignorant of such barriers and indeed produces new genes altogether whose natural barriers, if any, are unknown.

With CRISPR/Cas a new proliferation of genetic engineering has started; given how ignorant we are about the foundations of all life the chutzpah with which we tamper with it is breathtaking. I would actually be astonished if no catastrophe of some kind happened within our lifetimes. Some unpredicted horizontal gene transfer, some anti-reproduction measure which failed, a little lab escape. A sudden mutation which renders the must-have intelligence enhancing gene we have put in our children deadly. Shit happens.

Because genes are self-reproducing and spread easily they are an ideal agent to cause a mass extinction which could either target humans directly through an infection or destroy (read: dramatically alter to our disadvantage) whole ecosystems.

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  • $\begingroup$ I feel like you're a bit paranoid about this issue. While there is a lot of genetic engineering going on, there are sooooo many regulations on this field that the chances of anything like this occurring are super low. (Source: I work in a biology research lab and have a Master's degree in bioinformatics) $\endgroup$ – Whitehot Oct 9 at 9:56
  • $\begingroup$ @Whitehot I must be because few others appear to share my concerns. But shit happens. I suppose it's fair to say that we do not fully understand what we are doing, and the risk is enormous. $\endgroup$ – Peter - Reinstate Monica Oct 9 at 11:39
  • $\begingroup$ @PeterA.Schneider No. You are just more objective than most. Nature tends to ringfence nasty outcomes, rather than removing them. As we leap over the fences that nature provides we can expect to sooner or later leap into a corral that has been waiting for us. Alas. See my answer - which addresses the Australian mousepox experience. Do we really think that the worlds "bio warfare" people did not notice this, and that they have not been beavering away to "perfect it" in humans ever since? $\endgroup$ – Russell McMahon Oct 9 at 12:03
  • $\begingroup$ @Whitehot I have had discussions (long ago) with a person who worked in a secure facility with multiple levels of containment / protection against biological release. What they told me confirms my expectations of how such protections will come to be regarded and dealt with when decade after decade there are no bad outcomes seen, no problems arise, What can go wrong ...? || Does your lab have high integrity containment facilities that rely on people obeying rules and following procedures? Does it always happen? |The last western Smallpox death was 'caused' by an international class researcher. $\endgroup$ – Russell McMahon Oct 9 at 12:08
  • $\begingroup$ @RussellMcMahon when was this death caused by a lab error? I can't find it readily available $\endgroup$ – Whitehot Oct 9 at 12:51
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Create an organism with

  • infectiousness of the common cold,

  • payload when triggered like Ebola (mechanism may differ but Ebola is "pretty effective"

  • Silent transmission like HIV.

It doesn't trigger until we all have it and then it presents an Ebola like worldwide effect with nobody to help those who may otherwise survive with proper care. Charming.

Y'All die, y'hear.

It very likely already exists.
A few decades back Australian scientists came close to making a excessively effective mouse destroyer by genetically engineering Mousepox, but lnfectiousness was low - just as well for mice and humans.

One can be sure that the major powers took it from there and by now have a human targeted wipeout weapon too terrible to use.
You just need to posit its accidental escape. Or purposeful release by humanity-zero type advocates.

Australian mousepox development:

Populist outline{Guardian} - Lab creates killer virus by accident

7 page open access paper here The mousepox experience

Legion - web search.


Added:

ksbes: Unfortunaly(?) death rate for pox and ebola virus is not that large - about 1/2 to 1/4 would survive. We need to guarantee no more than 1/10 survival rate - more than twice lower.

Unfortunately - You miss my point, and you are incorrect factually. It would be good if you were correct under the conditions that I propose :-(.

Smallpox untreated is (was) horrendously lethal.
The Australin experimenmters (did you read any of my linked material) 'upped the ante' - they took benign mousepox and converted it to a killer that would have wiped out much of the worlds mouse population IF it had escaped and IF it had been more infectious. Labs worldwide will have been working on that for about 20 years with human payloads. The end result will be about 100% fatal - especially if EVERYONE has it.

Ebola when treated with the best case available and a significant death rate amongst the medical staff has a 25% survival patient rate.

This is what you do to try and survive while treating patients with Ebola

If EVERYONE has Ebola, or something of equally lethality AND disabling effect, then the survival rate will be minimal.

enter image description here

Photo: Super Hero in action.
Medicins sans Frontiers worker.
Liberia Ebola outbreak 2014-2015
About 25% of patients survive.
14 MSF members died

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  • $\begingroup$ Unfortunaly(?) death rate for pox and ebolavirus is not that large - about 1/2 to1/4 would survive. We need to garantee no more than 1/10 survival rate - more than twice lower. $\endgroup$ – ksbes Oct 9 at 10:46
  • $\begingroup$ The thing with lab created viruses, particularly ones in published articles like the Australian Mousepow that you mention, is that it's very easy to then create a vaccine for it, which in a crisis situation can be quickly deployed to immunise most of the world. In addition, the characteristics you listed for the "perfect" virus all come from different viruses, because they're generally incompatible. Even just having a silent yet infectious virus is very hard because as soon as the virus multiplies to infect others, the body will react and there will be symptoms. Not viable in my opinion. $\endgroup$ – Whitehot Oct 9 at 12:55
  • $\begingroup$ @Whitehot - maybe the vaccine goes awry? $\endgroup$ – IronEagle Oct 9 at 15:56
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    $\begingroup$ @IronEagle you can imagine plenty of complications in this scenario that could make it work, like ineffective or contaminated or hijacked vaccines. An epidemic at the same time as a war and at the same time as global power cuts and at the same time as a volcanic eruption would probably do the trick, but adding too many cooccurring events would be a bit suspiscious. $\endgroup$ – Whitehot Oct 9 at 15:59
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If you want to go for a tech related catastrophe, check out this article on the future of AI:

https://waitbutwhy.com/2015/01/artificial-intelligence-revolution-2.html

Basically your biggest problem isn't a terminator-like humanoid, rather a super intelligence with a single task at mind. If we die this way it's a pretty cool way to die.

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Megavolcanos like Yellowstone and Lake Toba: In the case of Yellowstone the richest nation of our current era would lose half of its territory and it's most fertile farmlands. Even ignoring volcanic winter (and that can't be ignored) the food markets will be chaotic for decades as one of the major food exporters go from exporter to importer. Any shock like bad weather, political crisis, in the remaining major producers would swing the food price very high, causing riots in the big cities of the industrialized nations and starvation in the less developed nations. These riots would bring revolutions and counterrevolutions that would wreck many nations (just look what happened to Syria), furthering the infrastructure damage done by the eruption. Some of these revolutionary/counterrevolutionary nations would wage large scale wars against each other, like the Nazi-Soviet war in WW2, increasing the damage. And that's just taking into account the damage to american farmlands, ignoring the volcanic winter.

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