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In about six months time every human on Earth is infected with a virus. How would a virus spread to cause such a quick transmission rate to infect 7+ billion people?

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    $\begingroup$ I suggest you try the game Plague Inc. Your goal is to infect and kill everyone on earth. I think you'll learn a lot while trying to win $\endgroup$
    – Martijn
    Jan 10 '20 at 14:35
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    $\begingroup$ Getting literally "every human" is near impossible. Infecting people in isolated environments (nuclear submarines, ISS, experimental self-contained biomes) is going to be near-impossible in your time period. Realistically, you can asymptotically approach 100% infection, but your rate of infection is probably going to look like a skewed 'S' curve; starts slow, speeds up, then slows down again as you get closer to 100% infection. $\endgroup$
    – Matthew
    Jan 10 '20 at 17:36
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    $\begingroup$ Out of the box (not entirely serious) answer: Death itself is a disease (not a virus though...) It has a 100% mortality rate. We are all already infected from birth, which makes it hereditary I guess, in that essentially all living beings are afflicted with it. (Possible exception: Jellyfish) There is no cure (until the Singularity occurs if that ever happens). All we can do is delay its progression. $\endgroup$ Jan 10 '20 at 17:39
  • $\begingroup$ There's also the possibility of some people having a natural immunity to the virus. They might be able to contract it at some point, but their body would destroy the virus by the time everyone else was infected, so they would no longer be infected, even though they once were. $\endgroup$ Jan 10 '20 at 17:56
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    $\begingroup$ @Martijn Madagascar... Grrr. $\endgroup$
    – Almo
    Jan 10 '20 at 18:39

11 Answers 11

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First, the virus has virtually no symptoms. If there are symptoms, we notice. If it does have any symptoms, they have to be sneaky, like increasing our sweat just a bit, or making a person slightly more needy of social contact. This probably means it hijacks and suppresses the immune system to some degree.

Second, the virus pretty much has to be airborne, shared by breathing. That’s the easiest transmission method. Sexually transmitted diseases just don’t get around (ironic). Skin contact diseases do spread, but not in casual interactions.

So, you’re looking at an airborne measels-meets-AIDS combo. Yuck.

But even with high communicability, you still have distribution problems to remote islands, Arctic and Antarctic climates, etc. I strongly doubt you can get every human within six months unless you have someone consciously spreading it. An organization playing courier will be needed to get it around that fast.

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    $\begingroup$ If you are happy with 99% of the population, or even 99.9%, there's always that trope of releasing it in a busy international airport. $\endgroup$ Jan 10 '20 at 16:28
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    $\begingroup$ @MatthieuM. or the Port of Madagascar if I learned anything from Pandemic. $\endgroup$
    – IT Alex
    Jan 10 '20 at 16:43
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    $\begingroup$ It could be much more effective at reaching remote areas if it's communicable in animals -- such as birds. Birds are everywhere, so even people in far flung corners of the Earth are susceptible. Moreso if the viral spores can get into the birds' fecal matter and can get airborne again through it. $\endgroup$
    – bvoyelr
    Jan 10 '20 at 16:50
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    $\begingroup$ I think the term you're looking for is "incubation period". $\endgroup$
    – NotThatGuy
    Jan 10 '20 at 19:24
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    $\begingroup$ @Almo No, he definitely means Pandemic. The sequel was released 4 years prior to Plague Inc, but was never developed for the mobile market. It's almost a direct rip of Pandemic. $\endgroup$
    – Anoplexian
    Jan 10 '20 at 21:30
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SRM covered a lot so i won't repeat the lack of Symptoms or the airborn part but i do want to expand on his

someone consciously spreading it

Ever heard of the Blood Plague Incident? IT was in the MMO World Of Warcraft where a "disease" got spread outside of where it was supposed to stay. This caused major community hubs to get infected fast with a high mortality rate. Problem is people wanted to avoid that so they went to more remote locations, with the virus following due to infected people trying to go there too (To intentionally spread it because of the "If i die we all die" mentality or just seeking refuge).

So that would be the reason remote places get infected, people fleeing or spreading out of s sense of "fairness". So for it to work he disease should be spreadable way before the it is detectable.

A likely scenario would be people fleeing infected areas, some of them are carriers (hell maybe even all of them) who think they are not sick and view the original inhabitant as selfish/paranoia and justify them taking control (and spreading the disease or at least kill the healthy) out a sense of need.

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  • $\begingroup$ This works only if infection is deadly and there is a reason to go away from it. Real world is not a game and we have quite cruel but effective methods of managing deadly infection spread (closing border, forbiding any travels, killing on sight, etc.). $\endgroup$
    – ksbes
    Jan 10 '20 at 7:00
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    $\begingroup$ @A.bakker of course you wouldn't go to Madagascar. You wouldn't be able to $\endgroup$
    – VLAZ
    Jan 10 '20 at 7:59
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    $\begingroup$ @VLAZ, never heard of people leaving due to cold epidemia. We have here this twice in year, but maximum we got - closed schools and people with masks everywhere. And flu never were able to infect more than 10-20% of all people, it still half of million death annualy worldwide $\endgroup$
    – ksbes
    Jan 10 '20 at 8:04
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    $\begingroup$ IIRC, the key to the Blood Plague was that NPCs were carriers but immune (non-combat health regen was high enough). So once the plague got established in a city there was no getting rid of it. $\endgroup$
    – Separatrix
    Jan 10 '20 at 8:33
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    $\begingroup$ @A.bakker, So we have a perfect "reality check" situation! Will see how it goes. $\endgroup$
    – ksbes
    Feb 26 '20 at 11:50
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Major International Events

The first cases appear in a city shortly before it hosts the Olympics, World Cup, or a similarly-sized international event, and many infected people are working the event in positions with lots of public contact (ticket-takers, concessions, athlete wranglers, etc.) Fans from all over the world attend and pick up the disease. The fans spread it further as they journey home, including many long flights with stopovers at major international airports as well as trains, buses, etc. Athletes pick it up and, being very fit and healthy to start with, don't show symptoms while they do press tours, public meet-and-greets, parades, etc. upon returning home.

By the time anyone's showing symptoms a few weeks later, it's all over the globe and spreading rapidly.

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    $\begingroup$ To give credit where it's due, and at the risk of spoiling a book old enough to buy alcohol in the US, this is basically the bad guys' plot in Tom Clancy's novel "Rainbow Six" using an engineered super-Ebola. $\endgroup$
    – Salda007
    Jan 10 '20 at 9:11
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Use birds as (additional) carriers.

As others have pointed out, some small pockets of human civilization might be completely isolated (though personally I doubt that they don’t have any (in)direct contact). But I’m pretty sure all of them have some contact with birds. The only possible exception is the Amundsen–Scott South Pole Station which is completely isolated for seven months between February and October. You’d have to time your virus correctly to infect them before the last flight in mid-February.

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    $\begingroup$ I was going to suggest dogs, cats, mice, rats, and pigs. A lightly-symptomatic disease that can travel among all mammals will travel all over. $\endgroup$ Jan 10 '20 at 18:32
  • $\begingroup$ This answer could be greatly improved by showing how effective birds as carriers would be using the migration paths and timelines of birds. $\endgroup$
    – NotThatGuy
    Jan 10 '20 at 19:31
  • $\begingroup$ @NotThatGuy: They only have to bridge the gaps between otherwise isolated human populations. Long-distance propagation of the disease can be accomplished by humans. $\endgroup$
    – Michael
    Jan 10 '20 at 20:29
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If you really aim at infecting

every human on Earth

that's highly unlikely, bordering the impossibility. There are human communities which are so far apart from other communities that the virus won't be able to reach them.

Think of the people living on Sentinel islands, or remote tribes in the Amazon forest, for example.

Our modern and high developed transport network makes it rather easy for a pathogen to quickly travel between continents, but then a capillary diffusion within the continent is another story.

It might be easy in Europe and North America, but for Asia, Africa South America, Oceania and polar regions it might take way longer than 6 months, if it can happen at all.

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    $\begingroup$ Yeah... I had already considered about remote islands and such which is why I asked on here. I figured airborne wouldn't survive long enough to travel and skin contact transmission would take forever to reach them. $\endgroup$ Jan 10 '20 at 6:40
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There is a theory that some viruses may originate from space (panspermia). So if you take the concept of a readily transmissible virus with minimal early symptoms (the "measles-meets-AIDS" virus suggested in other answers) and combine it with the idea of the Earth encountering a fairly broad cloud of virus-bearing material then you could start the infection process all over the place. Even small remote communities would see some initial patients, and the high transmissibility would account for spreading it to the rest.

I would probably not have suggested this answer if you had used the "hard-science" tag as while the panspermia hypothesis is credible enough for a significant number of experiments relating to it to have been carried out by NASA, ESA and maybe others, the results of those experiments appear to quite significantly constrain accidental panspermia, albeit not entirely rule it out. Essentially you need meteorites at least 1m in size to carry the micro-organisms in order for them to survive atmospheric re-entry. Therefore, for it to happen everywhere at once and not to have happened before you need something like a meteor storm but you can't rely on the existing annual meteor events. You probably need to invoke something like a new comet to dump a whole load of suitable rocks bearing the virus into Earth's orbital path. So it depends whether you want your plot to be fully "hard science" or whether you're happy with a "plausible but not likely in practice" explanation. Of course, depending on your plot you might have the option of deliberate panspermia, where the virus is seeded across the world by aliens.

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  • $\begingroup$ If you're willing to involve aliens, Ringo did exactly this in Live Free or Die. The Horvath dropped infectious agent (spores?) from a satellite in a polar orbit, which exposed the entire surface of the planet. Good for getting those "hard to reach" places, although people on nuclear submarines might still escape. (Ahem. Spoilers. Sorry; can't really name the book in this context without it being a spoiler.) $\endgroup$
    – Matthew
    Jan 10 '20 at 17:34
  • $\begingroup$ OP said six months time, so each submarine is likely to take at least one infected person aboard before the deadline. $\endgroup$
    – WGroleau
    Jan 10 '20 at 19:47
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You make it generally harmless, and spread through common animals that aren't seen as pests. Sounds like you want to read about Toxoplasmosis

Up to half of the world's population is infected by toxoplasmosis, but have no symptoms.[7] In the United States, approximately 11% of people are infected, while in some areas of the world this is more than 60%.[3] Approximately 200,000 cases of congenital toxoplasmosis occur a year.

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The virus spreads rapidly. We ourselves spread it. If we are infected by a virus and we meet someone, we cause them that virus too. But sometimes it is not the virus in my view. It is the environment that is polluted. And pollution causes harmful viruses. And of course, it will infect, everyone living in that environment. An example is the Holi festival in India. On Holi, everyone plays with colors that are toxic. So the color rises up into the air and everyone breaths in it. So almost everyone is infected with some respiratory issues or throat infections due to the presence of the fatal virus in the air.

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  • $\begingroup$ Toxicity is very different from a virus. Chemicals don't increase their amount if spread from one person to another, but dilute. Viruses, when spread, grow in the new person infected, so that the total amount of virus increases, the amount being very roughly the same per person. $\endgroup$
    – puppetsock
    Jan 10 '20 at 19:45
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    $\begingroup$ Pollution doesn't cause harmful viruses, but eg. water polluted with human waste is often full of them. Polluting more water won't help spread a disease around the world, because places with better sanitation and water purification will remain more or less unaffected. $\endgroup$ Jan 10 '20 at 20:27
  • $\begingroup$ Hello, Diana. Welcome to WorldBuilding. Although we encourage out of the box thinking for WB questions, this appears to not answer the question being asked. As others have pointed out, toxins are not viruses, nor do toxins give rise to viruses. You've presented a scenario for getting everyone sick -- a world-wide celebration -- but that doesn't enable a virus, and there is a lot of things a virus can do that toxins cannot, so that's deeply relevant to the question. Please be careful to stay focused on the question asked! It is easy (for all of us) to wander into brainstorming on WB. :-) $\endgroup$
    – SRM
    Jan 11 '20 at 1:16
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Thing is there are tens of thousands of medical facilities performing blood-tests every minute around the globe. The existence of the virus would be discovered early on and people would start contingency operations. Bunkers, quarantine facilities, etc. People are resilient and inventive.

The only way this could work would be the collapse of the civilization as we know it before the virus dissemination is started. The whole of the humanity going back to middle-ages, but even then you can't rule out bubbles of the old society living isolated underground or in hidden areas, some of them military protected and others going to great pains to remain hidden and unknown. There are people right now concerned with this exact scenario you are trying to concoct. I'm talking survivalist, be-prepared groups active on message boards, with tons of information regarding how to outfit your bunker best :) The downside of the civilization collapse would be that you'd have fewer ways to spread the virus (no planes, trains, cars, mails).

So you'd have to operate from space and not rely on the existing infrastructure. You'd have to have access to vast amounts of resources, deploy pods from outer space in a systematic manner.

And still I don't think it's possible to infect the whole of the humanity in its entirety.

Leaving aside the difficulty of reaching purposefully isolated and hidden groups living in bunkers or other hidden facilities, individual humans are unique at genetic level. There would still be people immune to the virus. You just couldn't get them all sick.

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I am not a specialist but heard about a real virus that seems to achieve what you want: It's name is "Papillomavirus".

It is a virus transmitted through genital zones, as well as through mouth. Thus, it is supposed to have infected every men and women in developed countries after 25 years. The thing is that you don't have symptoms, and no directly related diseases as well for most people, so the virus is not detected.

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  • $\begingroup$ cdc.gov/std/hpv/stdfact-hpv.htm "HPV is so common that almost every person who is sexually-active will get HPV at some time in their life if they don’t get the HPV vaccine." That's subtly but importantly different from "it is supposed to have infected every men and women in developed countries after 25 years." $\endgroup$
    – RonJohn
    Jan 11 '20 at 19:52
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You can have the disease actually change people's behavior so they want to get away anyone they know and towards people they don't know. This is similar in concept to how the parasitic wasp infects caterpillars- caterpillars protect the wasps (having been inside and eating and affecting the caterpillar) that burst from their skin with their own silk and attack any creature getting near the wasps.

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