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Inspired by this question, and my own answer to it: Avoiding galactic pandemics. World ending pandemics are a common disaster/after-the-end story trope, I'm trying to determine if plausible, and how one could occur.

What is the threat of a pandemic occurring in a modern first world nation, and if one is plausible then what conditions are required to make it occur? Keep in mind I'm talking about trying to stay to the realm of realistic science, in particular only diseases that plausibly could evolve in the first place.

I define a pandemic for this question as:

  1. Naturally evolved, or at least not intentionally man-made. I'm willing to accept diseases that evolved due to accident's or mistakes made by man, such as breeding antibiotic resistant bacteria via overuse of antibiotics, but no weapons-grade engineered diseases.

  2. Disease must spread for at least half a year without being contained

  3. Has a mortality of at least 10% (ie kills 10% of total population)

  4. Is a threat to modern-day first-world countries that aren't willfully ignoring the threat (like some African countries are doing with AIDS).

How plausible and what is the threat of such a pandemic?

Edit: Just to clarify, my interest is not in what sort of virus could be a threat, but how realistic such a virus existing would be! I am particularly looking for information about the likelihood of any suggested virus actually occurring.

Most relevant, the 'ideal' virus is one with a long incubation period and one that is not obviously lethal until long after symptoms are seen. However, I believe the more lethal the virus the shorter the incubation and quicker the time between symptoms and death would be. So if a virus must have a long incubation to be dangerous, I would appreciate at least addressing the likelihood of a long-incubation virus evolving.

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This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

  • $\begingroup$ Can you define "first world"? And are you expecting the name of the virus and/or conditions that may lead to its occurence, or mostly the characteristics of a completely hypothetical virus? $\endgroup$ – bilbo_pingouin Jul 1 '15 at 15:06
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    $\begingroup$ @bilbo_pingouin en.wikipedia.org/wiki/First_World :P $\endgroup$ – dsollen Jul 1 '15 at 16:13
  • $\begingroup$ @bilbo_pingouin specific virus would be great, but seems doubtful that you can provide. characteristics are fine, but in particular I would like to know not just characteristics needed, which I could guess, but how likely it is for such characteristics to naturally occur. The question is not rather a virus could be invented that was dangerous, but how realistic is is for said virus to exist $\endgroup$ – dsollen Jul 1 '15 at 16:15
  • $\begingroup$ I am wondering about a scenario similar to smallpox when Europeans met the Native Americans. If, say, 10% of the world population can be carriers without presenting any symptom or ill-effect, I would imagine the chance of the virus/disease spreading to be much higher. $\endgroup$ – Matthieu M. Jul 1 '15 at 17:54
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    $\begingroup$ You might find the answers to a similar question over on Quora interesting: quora.com/Infectious-Diseases/… $\endgroup$ – Mike Nichols Jul 1 '15 at 22:08
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A pandemic of the size you asked about is a credible threat. I've seen articles over the last couple of years where researchers attempt to model the spread of diseases and what affects how quickly they spread. The US CDC spends a lot of time thinking about this kind of thing. Given the diversity of research institutions who are working on understanding the spread of disease, I think the threat of a pandemic is a very credible.

The problem with short gestation period diseases is if you, as the disease, kill your host too quickly then you don't get a chance to spread yourself around as much. Diseases that kill quickly typically turn into diseases that don't kill too quickly because a fast-killer doesn't "survive" long in the population while tamer mutations have more time to expose other hosts. A virus will change over time in relation to the selection pressure of a host's immune system (herd immunity may play a factor too). The longer a virus survives and the more hosts it infects, the greater the chance for mutation.

There's this article from 1999 that talks about mutation rates in RNA viruses. (I'm sure there are newer, better articles to be had.) The mutation rates are stupid high on the order of around 75% per genome per replication. The mutation rate is much lower for DNA-based microbes (including both viral and cellular organisms) at .34%. The authors of the paper state that these numbers are hard to gauge because the mediocre quality of the data. If that's just one nucleotide involved in the mutation, at trillions or quadrillions of replications across many different hosts, there will be mutation.

Older Answer to the Question The ideal pandemic agent might have the following characteristics: Make the virus airborne, highly contagious (requires very small doses for infection), contagious shortly after infection, and a long gestation period in the months or years range before it induces acute illness/death. Basically, airborne HIV.

Virus based diseases such as HIV, Ebola or influenza have historically been very difficult to study based on how long it takes for effective treatments to appear after a virus discovery. We've known about HIV since 1980 but it took years to develop effective treatments. If this is a brand new disease then it will take a while for medicine to learn how it works and develop an effective treatment thus giving the disease plenty of time to spread. Ebola treatments are relatively new but medicine has known about it for decades (the Hot Zone was published in 1994). While the pandemic doesn't strictly need to be virus-based, having it be so decreases the changes of rapid effective analysis/treatment by the CDC or local equivalent.

Let's assume the following characteristics of this virus:

  • Average gestation period: 6 months, standard deviation of 2 months.
  • Illness Rate: 50%
  • Fatality Rate: 10% (per your question)
  • Required dosage for infection: 100 viruses.
  • Time to contagious: 3 days, standard deviation of 1 day.
  • Symptoms: Flu-like, coughing, aching, runny nose, headache, fever.

Plausible Scenario: Disease originates in sub-Saharan Africa (sorry Africa). Patient Zero catches the disease from infected pig meat. The virus causes minor inflammation in the nasal cavities, thereby inducing sneezing. It is passed to a foreign aid worker who is about to finish their tour and go home to California after stopping off in London and Paris to visit relatives. The aid worker sneezes on the plane without covering her nose (shameful). The water particles from her sneezes spread the virus to everyone on the flight. In 2 to 4 days, all those travelers will infect the people they know. Even if a person sneezing in a huge airport like Heathrow only infects 10% of the people moving through then that's thousands or tens of thousands of individuals infected. And, people flying through Heathrow go literally everywhere on the planet. People working in the airport will contribute greater still to the infection rates as they have a persistent presence to keep recontaminating the airport.

After stopping off in Paris and London, she heads home to California. The virus is still undetected and unknown. There are no quarantines for infected individuals, no blockades of infected countries. Assuming she has layovers in JFK in NYC and arrives in LAX, two of the largest airports in the US are now infected.

I think you can see where this is going. If nothing else, this kind of a disease is a special threat to the First World because of how interconnected those countries are and the common use of air travel. The power of the CDC can't help you if the virus is unknown.

Compound the effects of this new virus by having the virus start spreading in May/June/July. The 6 month gestation period will coincide with flu season and that will complicate treatment because doctors will think that people are getting sick from the flu, not this new virus. It will appear to the public as a terrifying flu season because people are getting sick and dying from the flu or something like the flu but the flu shots aren't working. The usual methods of quarantine for flu patients won't work because practically everyone was infected months earlier.

What makes this scenario plausible is the high degree of interconnectedness in the First World because of air travel. It's an epidemiologist's worst nightmare to have a lethal, airborne virus that gets spread around by air travelers coughing and sneezing on planes.

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  • $\begingroup$ same question I asked to the other's though, how plausible is a 6 month gestation period? I'm not saying it's impossible, I don't know enough about the biology to say for sure. but en.wikipedia.org/wiki/Incubation_period suggests the vast majority of diseases have short gestation, and generally I expect the more lethal the disease the shorter the incubation period. $\endgroup$ – dsollen Jul 1 '15 at 16:12
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    $\begingroup$ @dsollen I'm not familiar with the biology either and I wasn't aware of that Wikipedia page. I chose 6 months to ensure maximum infection rates before the normal disease prevention measures taken by society kick into play. Also, illness doesn't have to be caused by the virus itself but could be a secondary effect of infection. HIV doesn't kill you, it just makes it easier for everything else to kill you. $\endgroup$ – Green Jul 1 '15 at 16:26
  • $\begingroup$ This virus could attack your respiratory system, like flu, but in 10% of cases it attacks your liver and kidneys and the failure of those organs is what kills you. $\endgroup$ – Green Jul 1 '15 at 16:31
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    $\begingroup$ Direct flight from JFK to LAX? Our aid worker is coming in at a weird time of day, getting home just in time for Christmas, and can't afford the best flight, so she takes what she gets... she's got layovers in Atlanta and O'Hare ;) $\endgroup$ – KRyan Jul 1 '15 at 17:47
  • $\begingroup$ @KRyan, fair point. And it makes the scenario all the scarier. $\endgroup$ – Green Jul 1 '15 at 17:49
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The Spanish Flu came close to your specifications, with a few qualifiers.

  • It was almost a century ago. The state of the art has progressed since then.
  • It was global. Aggregate worldwide numbers mix industrialized and underdeveloped countries.

That being said, the Spanish Flu came within shouting distance of the scenario you asked for. So a similar pandemic, slightly more lethal, slightly more infectious, would fit the requirements.

Consider how legal and illegal immigration bring people from the "second" and "third" world to the "first" world. Say the fictional disease has a longer incubation period, and an outbreak in Central America or Africa would trigger mass refugee movements. How soon could and would borders be sealed? The Israelis in the movie World War Z allow uninfected refugees in to reduce the number of potential infected outside -- cold calculation and compassion working hand in hand. The border defenses will always leak ...

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  • $\begingroup$ but the real issue is the 'long inucbation period' concept. Yes diseases have different incubation periods, but what are the odds of lethal disease that also has an absurdly long incubation period.? $\endgroup$ – dsollen Jul 1 '15 at 16:05
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    $\begingroup$ just looked up additional statisics online, looks like 25% of the us and England caught the disease, and it had a death rate of 10-20% so 2.5 to 5% of the us population died. According to this source mortality rate was 2.5% in the US. Can't find exact mortality rates for england, but using those two as standards looks like mortality rates of 2.5 to 5% for first world countries probably. Also, antibiotics would have dropped it significantly due to pneumonia being the ultimate killer instead of the virus itself. Still a great example, I appreciate it! $\endgroup$ – dsollen Jul 1 '15 at 16:37
  • $\begingroup$ @dsollen: I believe that flu pandemic is what UK public health officials (that is, professional epidemiologists) treat as the most credible threat. Their opinions range from "it could happen any time" to "it will definitely happen from time to time, the only question is when". Whether the death rate in their worst-case scenarios is above or below your 10% threshold, after it's been mitigated by the measures they plan to take if/when it occurs, I don't know. $\endgroup$ – Steve Jessop Jul 2 '15 at 10:47
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    $\begingroup$ Also, note that flu is difficult to contain not because of a long incubation period (it's short), but because we're infectious before recognising the symptoms as flu. Your question specifies, "spreads without being contained for half a year", not "has an incubation period of half a year". Flu outbreaks are not completely uncontained, but they're usually not completely contained either. So for example the 2009 H1N1 pandemic killed at least 14k people worldwide, most of them on different continents from its origin in Mexico, despite attempts to contain it. $\endgroup$ – Steve Jessop Jul 2 '15 at 10:59
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There are three ways diseases can be highly lethal I can think of right away and one of those ways would take a long time to show symptoms. Basically, if the disease kills by causing gradual irreversible damage, the ability of the body to compensate for it would hide it until the amount of damage exceeds the ability of the body to compensate. Disease like this might take years to kill, it might even be difficult to connect the increase in deaths to a specific pathogen. This is because there would be lots of infected that would show no symptoms of the lethal disease and also people dying from the disease whose bodies had already kicked out the pathogen.

In a case like these there would first be a mild "flu like" disease spreading and after it was found the symptoms are no different than other seasonal flus, largely ignored.

Then after a few years people would start dying from a weird disease there some critical system starts simply failing. The first victims would be the old and those whose health was weakened by some other disease. There would be a large hunt for the reason. Maybe people are eating too much food additives? Maybe that new bio-diesel really was not that good an idea? Maybe those nuclear tests in the 50s and 60s permanently damaged the people born at the period? It would be difficult to home on to a single reason since realistically people dying in the first wave would be dying because of a combination of factors. If the pathogen was virulent enough it could be so common that noting it was present in most of the victims would be meaningless. And there would certainly be some cases where doctors would miss that the person had been exposed to the pathogen.

By the time somebody connected all the dots it might be possible for the majority of mankind to e infected and have sustained enough damage for that to be lethal. You could have a death toll exceeding 80 percent

The downside of this form of infection for fiction is that the 80 percent lethality might not make that much of a difference. It is easy to forget, but despite modern medicine nearly one hundred percent of people die. Often of some disease. So this kind of pandemic would simply have the effect of shortening the life span and giving developed countries, the first world, a population structure similar to one in the developing countries. In that sense it really would impact the first world much more. Which is a plus?

Exact effects depend on how fast it kills. If it kills slow unless you are already sick in some way, it would just change the population structure. There are people who complain about the direction the populations in developing countries are going, so this might not be all bad. If it kills everyone, no matter how healthy and young, within some relatively low number of years, you'd have a real pandemic in every sense.

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    $\begingroup$ NEARLY 100 percent of people die. BUT the remaining 0.1 percent... $\endgroup$ – Caridorc Jul 2 '15 at 13:26
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I am no specialist, but I think a pandemic must be based on a virus: it spreads easier, and harder to fight. Plague, e.g., can be countered with antibiotics.

Against viruses, we have currently two weapons:

  • Quarantine
  • Vaccines

Now, I think viruses have the following characteristics

  • Incubation time: time between exposure and symptoms,
  • Contagion time: time between exposure and when the affected person is contagious,
  • Propagation: means and effectiveness,
  • Fatality: number of victims succumbing to the virus.

We can then consider recent virus epidemic/pandemics:

  • Influenza H1N1: it is very contagious, the 2009-2010 reached 10-500 million people (more than 200 countries affected), and the 1918 (Spanish Flu) affected 500 millions. It can be transmitted through the air (coughs, sneeze), which facilitates its propagation. However its fatality rate is quite low (0.03% in 2009-2010).
  • Ebola: it is transmitted through direct contacts which limits its propagation. However the fatality rate is up to 70%. Even in 1st World countries (Spain, USA), some patients died from it in the on-going pandemic.
  • Influenza H3N2:the mutation of the virus "surprised" the experts, as a result of which many cases were observed this winter as part of the seasonal flu. It is considered somewhat deadlier. But I couldn't find any satisfying statistics.
  • Influenza H5N1: the avian flu is deadlier than H1N1, with more than 50% fatality rate. It mostly affects birds but can sometimes be transmitted to humans. The pandemic in 2006 affected a considerable amount of birds. But luckily for us the transmission from birds to humans is quite low.

Where does that lead us to find a killer virus? Well I think it can be seen that typical influenza spread very fast. But their incubation time is about 1-3 days (as per the link you shared). And their mortality rate is pretty low. By keeping people at home when the symptoms appear, the progression is limited, and due to good medical conditions many death are avoided (see below). Ebola on the other hand is quite slow. It cannot be transmitted by droplets from coughs and sneeze. And its incubation time is up to 20 days (or even more). However its fatality rate amounts to more than 50% even in medical centres.

Therefore, consider a deadly influenza mutation which increased dramatically the fatality rate, without lowering its transmission mechanism. You got yourself a very touch and deadly virus. Without going too far off in imagination. Is it enough to kill the close to 100 millions (10% of the 1st World)? I doubt it.

For most influenza case, we have vaccines to prevent them (if you do get vaccines). Otherwise, and for the other viruses, isolation/quarantine is the best option. As soon as a patient develop the symptoms, s/he is isolated from other people to limit the propagation of the virus. However, what would happen for an hypothetical virus, where people get contagious as soon as (or even before) the symptoms (fever) are noticed? Isolation would still be effective, but not as full-proof: neighbours might have been contaminated prior to the isolation.If the propagation medium isn't too restrictive (unlike, e.g. HIV and more like influenza), the quarantine approach would be limited. Which would still have a large spread (similar to the H2N3 of this year flu) in the 1st World. And if that virus is combined with a high fatality rate, you are in for a disaster.

This is as much as I see, without much prior knowledge and extrapolation.


But now consider an alternative scenario. In the 1918 Spanish flu, which was an Influenza A(H1N1), most of the victims did not die directly from the flu itself, but more the flu lowered the defence mechanism of the body, and they died, e.g. of bacterial pneumonia.

So just hypothetically, consider a strong H1N1 pandemic couple with a plague or other bacterial-based pandemic. That would probably be pretty bad.

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  • $\begingroup$ Very good answer. The only thing i would nitpick on is that we have very effective antibiotics for bacterial spread, so a flu like spanish flu would be far less dangerous today because we could treat the bacterial disease to control it long enough for patients to recover from the original flu. I suppose there would be a potential issue if we ran out of antibiotics because of the rate the flu spread. To a lesser degree antibiotic resistant bacteria's would spring up rapidly here, with little time to switch to new antibiotics as we now do when something develops immunity. $\endgroup$ – dsollen Jul 1 '15 at 22:57
  • $\begingroup$ @dsollen You are perfectly right, the level of hygiene combined with antibiotics make it that 1st world inhabitants are, on the whole, pretty safe. Now, if somehow the bacterial infection spread (say via common animals: dogs, cats) together with the virus, and due to different incubation time strikes during the isolation time with first symptoms not so different from the virus (fever, sore pain, etc), it would probably have a high infection rate. Now if the bacterial is resistant to (at least some) antibiotics, you're in for trouble. $\endgroup$ – bilbo_pingouin Jul 2 '15 at 6:42
  • $\begingroup$ Note that it might still be damaging "enough" for the first world of your scenario, but it would almost certainly prove disastrous, were it to spread to the third world. $\endgroup$ – bilbo_pingouin Jul 2 '15 at 6:43
  • $\begingroup$ so imagine Ebola being propagated via airborne droplets... half of Africa would be dead by now and there would be a continent-wide quarantine. I think Ebola as it currently stands would be an immense disaster if it broke out in a heavily populated city such as London or New York as it'd spread massively before anyone even knew it was here and the drug companies almost didn't bother to make drugs combating it . Maybe that'll be the next terrorist attack that succeeds :( $\endgroup$ – gbjbaanb Jul 2 '15 at 12:33
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    $\begingroup$ It would be pretty bad, but the transmission of ebola as we know it still prevents major disaters. "The potential for widespread infections in countries with medical systems capable of observing correct medical isolation procedures is considered low." as per wikipedia and reference therein. A good isolation, treatment and handling of deceased would probably limit the scale of the disease. Economically, it's another story. $\endgroup$ – bilbo_pingouin Jul 2 '15 at 12:44
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I had an idea about this. I'm not sure exactly how plausible it is, but it is inspired by hypomyces. The disease doesn't affect humans DIRECTLY, rather, infects and mutates other viruses. So, you take a virus which is difficult to treat, such has herpes which already infects some 90% of the population and the theoretical virus would be easily overlooked because it has no effect on human tissue directly, instead causes herpes to become a cancerous flesh-consuming blight. makes me shutter.

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    $\begingroup$ This does not seem plausible. Firstly, viruses are incredibly sloppy replicators already so higher mutation rates are unlikely to help and, secondly, viruses - as a class - cannot be generally 'infected'. There are parasitic viruses that hitch-hike with other viruses but that isn't much like you'd need for what you describe. $\endgroup$ – Jack Aidley Jul 1 '15 at 21:21
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    $\begingroup$ Perhaps a more viable derivation of this is that rather than directly infecting humans, it infects the microbiota that lives in and around us. For every "human" cell, 10 bacteria live in and on us. If the virus infects those and causes those to turn from probiotics to harmful pathogens, it might take a while to figure out the new harmful bacteria that is harmful to people but we can't find anywhere but other people and doesn't seem like it's spreading. $\endgroup$ – Jim2B Jul 1 '15 at 22:15

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