In the case of a world-wide plague epidemic that'll trigger the near extinction of humanity, is it possible that a virus/disease has inconsistent symptoms so that people aren't aware that it's really the same plague until it's already spread or in the later stages?

This is assuming that the plague either originated from one part of the world or perhaps started off in several places in individual forms before it mutated as one new strain once the right conditions were met.

Is this a possibility? Bonus points if you can explain how diseases/viruses begin in the first place or mutate and become resistant - whether the disease/virus is natural or manmade. :-)

Thanks in advance!

  • $\begingroup$ Taking a rough guess at what you are aiming at: Might a long incubation time do what you want? $\endgroup$ – Wrzlprmft Oct 8 '14 at 19:49
  • $\begingroup$ @Wrzlprmft When you say long incubation, meaning it takes time from getting infected to the symptoms becoming obvious that it's the same killer plague? That's fine, the faster the better though. :-) $\endgroup$ – KaguraRap Oct 8 '14 at 19:59
  • $\begingroup$ Yes and no: It takes that time for symptoms to emerge at all and a plague being obvious at all. So almost everybody might be infected before anybody notices that there even is a plague. However, when the symptons start to hit, it will be as obvious that it’s the same plauge as without a long incubation time. (Note that this arguably is one of the key features which made AIDS this successful.) $\endgroup$ – Wrzlprmft Oct 8 '14 at 20:10
  • $\begingroup$ Hmm you mean kind of like "The Walking Dead" premises where they realize even if they're not bitten, if they die, they'll wake up a zombie because they're all already infected and just didn't know it. And so even if people have different symptoms, it's going to take time and therefore time for them to realize it's a plague once the symptoms are already consistent - and by then, it's too late. Not sure if I made sense there... $\endgroup$ – KaguraRap Oct 8 '14 at 20:18
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    $\begingroup$ I strongly suggest that you read the Wikipedia article on incubation time. What I am talking about, is a disease whose latency period is shorter than its incubation period. People have no symptons for some time but are already infectious. As soon as the first people begin to show symptons, everybody is already infected. $\endgroup$ – Wrzlprmft Oct 8 '14 at 20:35

First things first, I am not a doctor - this is purely educated speculation.

Is this a possibility?

I would say that while it is possible it is improbable.

Let's start on the inconsistent symptoms part first. In known diseases, symptoms are largely consistent since the diseases impact the same systems from host to host since they're tailored to attack that system. No biology will spontaneously evolve to be universally successful at attacking different systems. This is the primary reason why I say it is improbable.

So what unknown diseases may impact different systems while still having the same sort of attack vector?

  • Transmittable cancer - Cancer is one of the few that does happen in multiple systems, even though the tumors themselves are the same sort of stuff. Assuming cancer was somehow contagious, the initial point of tumor development would dictate the symptoms.

  • Environmental variation - A disease could behave differently (prefer different parts of the body) depending on what the environment (heat, humidity, atmospheric content, etc.) is. For warm blooded creatures, I expect this to be unlikely since their environments are largely uniform. For cold blooded creatures, or creatures in weird diverse biospheres? Plausible. It would also yield "regional" diseases, that are more likely to be considered different.

  • Transmittable genetic disorder - The cancer may be a subset of this, but a disease that attacked random points of a creature's DNA would result in significantly varying symptoms - as well as a fairly slow development time.

That's part one of the unlikeliness. Part two is the transmission time.

Naturally developed (known) diseases are things that effect different species that mutate to impact the new species, or are existing diseases that mutate to cause different effects. In both these cases, you start with a single infection. Because symptoms are often caused by our immune system's response to the disease rather than the disease itself, you have very little time for that single person to distribute the disease before they show symptoms and become non-contagious by 1) being cured by medicine that works against the pre-mutated disease or similar, 2) dying and/or 3) being avoided by people.

Diseases can't really just "sit and wait" since they need to get energy from somewhere, which means leeching from their victim. That will be enough to trigger the immune response (and symptoms).

And the other part is that since diseases come only a short way from an existing disease, they're unlikely to be too successful at killing people, else they would burn themselves out.

As for biological warfare? It has far fewer restrictions. As do diseases introduced spontaneously to an area (think smallpox to North America).

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  • $\begingroup$ Marked as an answer for telling me even if it is possible, it's highly unlikely and also these lines: "In known diseases, symptoms are largely consistent since the diseases impact the same systems from host to host since they're tailored to attack that system. No biology will spontaneously evolve to be universally successful at attacking different systems." More points for providing real life examples of diseases e.g. cancer and mentioning how diseases start/end. Thanks! $\endgroup$ – KaguraRap Oct 9 '14 at 14:59
  • $\begingroup$ While not present in humans, there is such a thing as a transmissible cancer in Tasmanian Devils. Take a look into en.wikipedia.org/wiki/Devil_facial_tumour_disease $\endgroup$ – Erica Jun 12 '15 at 2:39

I don't really see biological convergence a possibility (several virus's that start off separately and somehow mutate to the same end result). A systemic virus could hit several sections of the body at once...if some people die because the effect on the kidney's takes them out first while others die because of heart issue prior to the kidney portion arising...still seems improbable.

I can see virus synergy as a potentiality possibility...several virus's that on their own display very separate symptoms but are not that deadly, however they become very deadly when combined in the same host. I might be stretching there.

I'm not sure on viral origins...but the general concept on them is one of highjacking. All cells have 'receptors' that can have molecules with the right configuration link in to that receptor (we'll call those keys). Virus's have millions of these keys and float around until it finds a cell that meets ones of it's keys (these 'keys' vary from species to species, which is why many virus's cannot jump across species). The virus uses that 'key' to get into the cell and locates the dna copy device that each cell possesses (I'm simplifying pretty heavily here). The virus then lines itself up to copy itself using the same technique your DNA uses to copy and presses the make a few billion copies button. The copies mass generate until they overwhelm the highjacked cell, which explodes, launching the copies out into the body to repeat the process. This creates an extremely high chance of mutation as each copy of that virus could be slightly 'miscopied' into something a bit different (basis of mutation)...perhaps one of these copies develops a 'key' that could infiltrate a new species for example (whether that key ever gets used is a different question). Or as an example to the question...it could accidentally mutate a slight resistance to a medicine that normally kills it. Medicine kills all the other instances of that virus, leaving only the ones with a fluky mutation making it resistant to the drug remaining...1 of those infects a cell and suddenly you have millions of copies of a virus that all retain some resistance to that particular drug.

As a side note...Our immune systems are adept at locating these virus's and clogging all of its keys that match our cells until the virus itself dies.

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  • $\begingroup$ The brief simplified overview of how viruses interact with cells gets an upvote! So if not different viruses ending up the same, I guess perhaps if one virus came from one part of the world and then another one from another part, both viruses travel to this one particular area and as you said, if someone contracts both viruses, perhaps it mutates as a new virus? I don't have any biology background - I'm making this up haha. $\endgroup$ – KaguraRap Oct 9 '14 at 14:55
  • $\begingroup$ "I don't really see biological convergence a possibility (several virus's that start off separately and somehow mutate to the same end result). " when you have a cold, can you tell whether it's due to a rhinovirus or a coronavirus, or whatever? The symptoms are the same for over 200 viruses in different families: the symptoms are due to the tissue infected, not the specific agent. That could apply to other tissues as well. $\endgroup$ – JDługosz Jun 12 '15 at 0:10

I'm afraid I must go against the crowd and say it's certainly possible. The thing is we have already seen a plague that's killed millions that had this behavior: HIV.

The key here is that the disease has no obvious symptoms of it's own, but rather makes you vulnerable to other problems.

A readily-spread version of HIV meets your criteria.

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It is very possible if your disease was not caused by bacteria or viruses, but a parasitic organism.

The symptoms of the condition would be highly dependent on where the organism ended up. It could range from strokes to liver failure to an inability to breathe, the possibilities are endless.


  • parasitic infections can be very hard to find, depending on the size of the parasite and how similar the symptoms are to other diseases.
  • it could look like a wide variety of diseases based on where the parasite ended up.
  • parasites are not very infectious compared to bacteria and viruses.
  • a lot of parasites I know of do not develop well outside of their favorite feeding ground in the body.

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Technically there is nothing saying a disease should be caused by same virus, so take 3 different viruses spreading at same time (Ebola or whatsoever) each one with the potential of reducing population to 60%, if they spread at same time you would get a combined effect disease with the potential of reducing population to roughly 21%. While at same time you would get combinatorially 7 different range of symptoms:

  • People infected by Viruses A,B,C would have all symptoms
  • People infected by Viruses A,B would have symptoms combined of viruses A and B

same applies to viruses combinations (A,C) (B,C) (A) (B) (C). Also it is possible certain symptoms could get combined into new symptoms (if one disease make your blood green and the other one make you bleeding you would have a green bleed).

There are documented cases: when a popolation is immune to a virus would get certain symptoms or no symptoms at all, if you expose a different popolation you would get different symptons and eventually death cases.

Also note that your "bold definition", could be applied to humans as well, not to viruses only (well, humanity is able to wipe out humanity in a range of ways in more places at the same time ^^ )

It could likely happens for a genetic modified virus:

  • Take any lethal virus (Ebola)
  • Farm the virus into different tanks
  • Than modify the virus with small changes to make it non-lethal and at same time weak (this is the hardest part)
  • The changes have to be different for each tank
  • Spread the virus world wide
  • The only chance for the virus to survive is to evolve, and the nearest evolution that it can do is to "revert" back to original status

Since it is weak it is going to "die" or to "evolve". That assume that the virus is constantly exposed to most of the population world wide requiring a governments mass action that is unlikely to happen (so even harder to happens in nature).

In all cases, if such things happen you would know that is not by any mean caused by "nature".

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Note my earlier comment on the great imitator.

Adding to the idea of multiple different diseases, perhaps there are several metabolic pathways that can compensate for each other, so knocking all of them out simultaneously is quickly fatal.

Similarly, some toxins work via secondary products. One diseased organ makes substance X, which another organ (or portion of) can eliminate, break down, or use up. If a different disease has knocked out that second process, then the first becomes fatal.

Now here's a more sinister variation: the treatment for one disease causes a second disease to be much worse or express unique symptoms. Only people in a community using medicine A will experience B as being of that particular form. Another community will see B as being a minor sniffle and not take special precautions to prevent spreading it.

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I'm going to insert something from reality (or at least my memory of a real document) - in this case a report from a researcher who was part of soviet bio-weapon research. Perhaps the model they developed will inspire...

Researchers started with a mild form of legionnaires disease. The symptoms are typically mild and flu like. It is easily transmissible for a long, several week window, after which the body's immune system fully eliminates the pathogen.

However, in this case, researchers genetically modified the legionnaires bacterium to express a protein taken from the myelin protein coating of rabbit nerve cells (insert your species of choice - presumably human).

So, this is how it plays out: Victim infected with modified bacterium. Becomes contagious, infects others. Becomes sick. Might go to hospital, but if he does, is diagnosed with mild flu or legionnaires, treated and sent home. Victim's immune system responds to modified bacterium and eliminates it. HOWEVER, the body's immune system has identified the invading agent by analyzing the proteins in the bacterial case, one of which is the myelin protein. The body's immune system now recognizes myelin as an invading agent. Over the next two weeks, the body's immune system mounts an assault on the myelin coatings of the body's nerves. First symptoms similar to multiple sclerosis appear, then closer to stroke, soon coma and death.

When these patients are in the hospital, no agent can be isolated from their blood. There is no apparent cause for their death that can be cultured from their blood, although it will be apparent to epidemiologists that this is somehow a contagious disease.

The Russian researcher said this was successful in rabbit test model. He was coy about whether this was ever tried in humans...

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If your disease is spread by self-replicating nano bots, totally.

Other than that, I would think that in the near future we'll be able to make things targeting specific DNA sequences and such. If your plague was engineered to behave differently depending on the DNA sequence observed in the host, it seems possible that it would present differently in various hosts. People with organ transplants would potentially make this harder to diagnose if the plague guessed wrong and triggered symptoms based on the transplanted organ(s).

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