I would like to know what would happen if a new virus arose which could not be cured and which managed to escape the infected areas (and any quarantine zones). In order to scope this question I'm not interested in how humanity would adapt I want to know about how the infected rates/death tolls over time based on patterns from historical epidemics.

  • Let's assume the virus is spread through body fluids (seems common enough)
  • No human doctor can treat it

I want to know

  • Assuming there was no medical cure would everyone die?
  • Or is it likely some people would be naturally immune/recover?
  • What would happen once the virus infected everyone? Would the entire population die off? Would it subside (like a predator) then reassert itself when humanity has rebuilt itself?

The answer I'm looking for will look at major outbreaks from the past and extrapolate what would happen if something similar happened today. Just to repeat myself I'm not looking for humanity's response to a global contagion, I'm looking for the patterns the virus would follow.

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    $\begingroup$ It is impossible to separate the question of how quickly it would spread across the world from the question of how humanity would adapt, because humanity would precisely adapt to slow down the spread. $\endgroup$ – celtschk Oct 28 '14 at 7:55
  • $\begingroup$ @celtschk that's true - I'll take that bullet point out $\endgroup$ – Liath Oct 28 '14 at 8:00
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    $\begingroup$ we have viruses like that, most of course aren't lethal killers. I think you might want to be more specific on what type of virus this is. We have quite a few VRE's that are dangerous, of course they are most containable. But the point is there are a lot of bugs we can't kill, partly because of poor use of antibiotics. $\endgroup$ – bowlturner Oct 28 '14 at 13:39
  • $\begingroup$ This really depends on how deadly the virus is. .....oh, I see someone made that point already. $\endgroup$ – Shokhet Nov 27 '14 at 19:12
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    $\begingroup$ Lethality (how fatal it is if you catch it), ease of transmission, virulence (the likelihood that it will hurt you if you get it), and how long the virus can live absent a living host will all play into this. (If a virus is too good at killing people it can die out before it spreads to everybody.) $\endgroup$ – Monica Cellio Nov 28 '14 at 20:13

10 Answers 10


First, the deadliness of a virus can vary widely from mostly harmless (Herpes) to very deadly (Ebola). However, note that even for Ebola, there are people who survive without treatment. Indeed, Ebola cannot be cured; all the treatment does is to try to keep the patient alive for long enough that the body can fight the virus by itself.

An important question is how quickly the virus mutates. Most viruses mutate slowly, and therefore if you survived once (or got vaccinated, if a vaccine can be developed) you're immune to the virus for the rest of your life, or at least for many years. Other viruses, like Influenza or HIV, mutate more frequently, which means that even if you're immunized against one strain, you'll still be unprotected to another strain (this is why Influenza vaccination is repeated every year: It's always a new strain that becomes prevalent).

Note that there's also an evolutionary force on the virus: As soon as it spreads too violently, it will have a harder time to spread further, since all potential hosts are already dead or immunized, and moreover will put the more effort into avoiding the infection, the more deadly it is. Therefore there's a selective pressure on the virus to become less deadly, or less infectious. Probably as long as there's no cure, ultimately an equilibrium would be reached like for measles or mumps in the middle ages, where it is common, but not as common to threaten humanity as a whole.

Of course, as soon as a cure of an effective vaccine is developed, the equilibrium will change to less ill people, if the virus will not be eradicated completely (as happened with smallpox).

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    $\begingroup$ indeed, NO virus can be cured. All treatment can ever do is slow down the spread of the infection so the body has more time to respond and fight it on its own. $\endgroup$ – jwenting Oct 28 '14 at 8:55
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    $\begingroup$ yes, there's a selective pressure on the virus to become less deadly, BUT MORE INFECTIOUS. From virus POV you want infect as many people as possible, and them to life as long as possible in infect more people. Killing host is unfortunate (all viruses inside dead host will stop spreading soon). $\endgroup$ – Peter M. Oct 29 '14 at 0:44
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    $\begingroup$ @PeterMasiar Actually some pathogens deliberately kill the host as part of their life cycle (Anthrax and Toxoplasmosis (en.wikipedia.org/wiki/Toxoplasmosis#Rodent_behavior) for example) $\endgroup$ – Tim B Oct 29 '14 at 10:04
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    $\begingroup$ @PeterMasiar - It depends on how corpses are handled: funerary rituals (Ebola benefits from that), decomposition through other life forms which become vehicles for virus. $\endgroup$ – mouviciel Nov 28 '14 at 9:07
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    $\begingroup$ Mutation rate is slightly more complex than you let on. If a virus mutates too fast, it will be incapable of holding on to beneficial mutations and will mutate itself out of viability. Research was being done to increase the mutation rate of HIV as a treatment because it is believed HIV is very close to the maximum practical mutation limit. $\endgroup$ – otakucode Dec 20 '14 at 8:48

The patterns viruses take is highly varied, which gives scientists little to go on. Major factors include:

  • How contagious is it?
  • How long is its incubation period, if any?
  • How deadly is it?
  • How well does the human body adapt to new versions compared to the virus' mutatability?
  • Did it intend to kill its host, or was that an accident?

These are not additive, so you can't just add contagiousness+incubation+deadly and look that up, they should be treated as independent dimensions, creating a hypercube of options.

For instance (glossing over a ton of details in each example for brevity):

  • Flu is highly contagious, but the human body adapts to it pretty quickly. Accordingly Flu spreads like a wildfire, but quickly extinguishes itself every year, falling back to its "flame keeper" in China where humans, pigs, and birds intermingle enough to keep things alive. Without that "flame keeper" producing new variants, Flu would not be a major issue today. As is, it is dangerous enough that humans make minor lifestyle changes to reduce its transmitability, such as masks.
  • The common cold is ubiquitous, so it never quite dies. There are a plethora of variants going around at all times, so its hard for the body to keep straight which ones it needs to protect against. It is also very non-lethal, more of a nuisance, so there is little pressure for humans to change their lifestyle to combat it.
  • Ebola is highly deadly, and very contagious, but it kills its victims too readily. Accordingly, Ebola outbreaks look like a small spark in gunpowder. It strikes quickly, hot (killing people on the way), but subsides quickly just the same, because it is so dangerous that humans will completely change our lifestyle in order to combat it.
  • Poliomyelitis actually lived alongside us for thousands of years without an issue, until sanitization decreased our ability to develop immunity to it in small doses and it became the Polio we know today. It struck people so hard and so fast that we actually crushed the virus under the force of our intellect.
  • The Plague traveled by rats, making it tremendously contagious, and we were highly susceptible to it. It basically ran unabated until it literally ran out of victims... the latest theory is that some portion of humanity accidentally had immunity to it, and that is the portion that reproduced to spawn the next generation.

It's only a game, but consider games like Pandemic 2 for more ideas that can should guide your research.


One of the best possible answers to your question is a stroll down history lane to the 1918 flu pandemic. Unlike most viruses, the Spanish flu checks off every box in your question:

  • could not be cured
  • managed to escape the infected areas (and any quarantine zones)
  • the virus is spread through body fluids
  • no human doctor can treat it

In the span of two years, it circled the globe twice, infected more than a half billion humans (1/3 of world population at that time), had a 10-20% mortality rate, and killed 3-5% of the world population at that time.

  • $\begingroup$ I was going to post a similar answer. Flu is an excellent example. Although it's usually fairly minor it mutates rapidly and crosses between just about all warm blooded species. So there is always potential for another very deadly pandemic to emerge. $\endgroup$ – nigel222 Apr 15 '16 at 11:58
  • $\begingroup$ There was a flu scare a few years back that may have illustrated mutation towards more infectious but less deadly. People were dying in Mexico but as the virus spread the mortality rate fell rapidly to "harmless" flu levels. A near miss? $\endgroup$ – nigel222 Apr 15 '16 at 12:02

A virus that kills people is not a human virus. The most deadly viruses are usually those who usually infected animals, but were somehow recently transferred to humans. It is not an aim of a virus to kill its host. The aim is to spread. As such, over time the most deadly deseases either become less deadly or humans selected who are not affected with it. This way an animal desease becomes a human one.


Small but important point here:

You cannot cure a person from a virus infection

Read that again: you cannot cure anyone from a virus infection. Bacterial infections? Yes, antibiotics will kill the bacteria and thus remove the infection. But viruses are a different thing.

Side-note: Are you still dousing your hands with disinfectants in hope of not getting a virus-born flu? You can stop now, because viruses do not give a hoot and a holler about ethanol and other disinfectants. They only work on bacteria. And you are very resilient towards bacteria that are on your hands. The hand disinfectant craze came because some people saw that hospital staff were doing this. But this is because hospital staff deal with people that are not resilient against bacteria, such as cancer patients that have no working immune system, which means that bacteria on your hands that are harmelss to you may very well kill such a patient. But for the average healthy human being, dousing your hands with the stuff (and smelling like you just downed a quart of vodka) is a sheer waste of time. Soap and water — thus physically washing away the virus before it gets inside you — is how you do it.

The only thing you can do is to keep someone alive while the immune system deals with the virus infection. You can also pre-emptively prepare the immune system to deal with the infection before it strikes (vaccination). You can also boost the immune system somewhat during an ongoing infection which may change the odds for the immune system to deal with it successfully but by then you are pretty much relegated to a being cheer-leading squad for own body.

From this your question becomes a matter of 1) how deadly is the infection once you get it and 2) to how many will it spread. If the answers are 100% and 100%, then the answer is: we all die.

Thus far we have never had such an infection in human history. We have "infections" that reach 100% of the population, but that are not deadly (gut flora for instance), and we have had infections that are 100% deadly (or nearly so) but that do not spread very far. So far we have not had anything that does 100-100.

Pray that nothing such comes along before we know how to keep a person alive no matter what they are infected with...

  • $\begingroup$ "If the answers are 100% and 100%, then the answer is: we all die." - really? Isn't there the danger that if the virus kills 100% of its victims, that it will burn itself out - i.e. kill all its potential carriers - before it gets us all? $\endgroup$ – colmde Jun 11 '18 at 8:45
  • $\begingroup$ @colmde For a fun answer to your comment, I can wholeheartedly recommend the game Plague Inc: Evolved. The short — no-fun — answer to your comment is: 1) If you play a scenario in Plague Inc. the way you just said, you lose that scenario. 2) No scenario in Plague Inc. is unwinnable. Therefore: no, you are wrong. :) $\endgroup$ – MichaelK Jun 11 '18 at 9:01

For an extreme case, look at what happened to the New World after contact with Old World diseases. It was a combination of diseases that did the damage but overall it was probably at least 90% of the population dead and some entire populations wiped out (it probably wasn't a 100% kill, but rather reducing the population to a point of non-viability.)

The source of your pathogen is very relevant. The more similar to things already out there the more likely there is to be at least some resistance (for example, cowpox providing protection against smallpox.) When the degree of similarity is low the lethality can be very high as we saw in the New World.


When does it become infectious compared to when does it take it's host out of circulation? The previously-mentioned Ebola, while highly lethal doesn't spread very well because it doesn't leave it's hosts walking around spreading the disease. (It's primary spread is from the sick and dead to caregivers.)


HIV, on the other hand, leaves it's victims quite capable of spreading the disease for a considerable interval after infection. What saved humanity from catastrophe here is HIV's lack of infectiousness. It has almost no ability to penetrate healthy skin and doesn't even fare very well against intact mucous membrane.

Now, consider something with HIV's ability to stay infectious and dormant for an extended period with something that copes with skin better (say, gonorrhea) and the Spanish Flu would no longer be the standard example of how deadly a disease could be.


First and foremost, everyone would not be infected, let alone die. Look at Brazil and Peru and the tribes of the Amazon, many of whom still haven't been contacted. Since they have no human contact, they won't get non-airborne diseases from outside. Since you say it is fluid-borne, they won't be affected. This can also be demonstrated by the fact that when new tribes are contacted, two-thirds of them die out within the year (Tribes of the Amazon documentary, BBC Four). This is because they have not had any diseases from outside, so are not immune to them.

It is unlikely to abate then resume. Many viruses have a life of just a few minutes outside the human body; even the more deadly viruses like Ebola and anthrax (though not technically a virus) have a lifetime of a few hours. For example, the Plague did this: killed off a load of people, then died out. Admittedly it was helped in London by the Great Fire in 1666, but in many other places it just died out.

In the end, it's likely that a significant proportion of humanity would be killed; modern communication and travel makes spreading disease much faster than in the past so an epidemic can become more deadly. However, some isolated humans would survive and might be able to make use of the remaining technology and machinery to survive.

  • $\begingroup$ (Almost) everyone might get infected if it were something like flu that infected multiple species including a common migratory bird. $\endgroup$ – nigel222 Apr 15 '16 at 12:06
  • $\begingroup$ A virus mutation which is less deadly and more infectious has an evolutionary advantage. If the milder infection conveys some immunity to the deadly strain then it will drive the deadly strain to extinction. This has been observed "in the wild". $\endgroup$ – nigel222 Apr 15 '16 at 12:12

Another point which seems to have been overlooked: Can this virus infect human germ cells? That is, does it infect sperm and/or egg cells? If so, it is likely that the virus would evolve to be less deadly (killing the host is bad for things which live on the host) and, eventually, it would integrate into our genome. Our genome, and those of various animals, are littered with viral DNA. Right now in Australia koala bears are facing this event. ALL koalas except a small population residing on Christmas Island are infected with a virus that is currently being integrated into their genome. The virus impairs their immune system similar to AIDS in humans and it may end up wiping them out if it does not become less lethal as it becomes part of their genome.


If you're going for extremes, go read The Stand by Stephen King. A genetically modified virus, the Blue Virus, is accidentally released from a military laboratory. Starting with one single family trying to flee, the thing spreads at a speed unheard of, with a 90% of fatalities. It goes so fast that media dub it 'Captain Trip' and leaves behind a world of dead. Only an incredibly restricted amount of people gifted with a natural immunity survive. Them, and some wild species. Cats, dogs, cows are affected as well... So you want a more extreme bad guy? Easy, insects survive, nothing else does.


People would turn against each other. It would replace all other divisions between people. Suddenly race, class, country, etc. would mean nothing in comparison to who is and who is not infected.

The whole of planetary society would become about who is infected and who is not. Children would turn in their parents. Tales would become about the brave infected person who took his own life or the villain who tried hiding it. People would accuse their enemies of being infected and society would be unable to handle distinguishing fake from real accusations. As such the mere accusation by anyone seeming credible would often be as good as a death sentence.

All of society would come to a crash. People would be afraid to go to work. The power would go off. There would be looting. The police would be replaced by, probably frequently religious, militias who would shoot a person on site if they didn't trust them. The virus would effect people on an individual level but it would also eat the fabric of society.


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