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The basic idea is that a small group of people, that were infected by the virus but managed to survive, has their hair colour changed to white and their eye colour changed to green. Said traits would then be passed on to future generations.

Some context info (changeable if needed):

1 The affected group is ethnically pure = physical traits vary less (think modern-day Japan).

2 Eye colour has changed from blue to green; if not green then gray.

3 These traits aren't dominant (this comes from my very crude understanding of genetics); a kid would need both of his/her parents to be affected by the virus to be fully white-haired/green-eyed.

4 The virus has a kill-rate of 90%.

5 The world is comparable to Europe's late Medieval Era.

6 There's no magic.

(Edited):

7 The virus is transmitted through the air.

8 It is naturally occurring.

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    $\begingroup$ Welcome to the site scipio, please take the tour and read up in our help centre about how we work: How to Ask Could you tell us about the way your virus is transmitted and it's physiological effects and what sort of virus it is - Baltimore Classification would be fine: en.wikipedia.org/wiki/Baltimore_classification My point is, at the moment your question is unclear and non-specific, therefore it is also too broad. $\endgroup$
    – Escaped dental patient.
    Mar 27, 2019 at 23:00
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    $\begingroup$ Possible? Yes, obviously: eye color and hair color and indeed (mostly) genetically determined, so it is possible that an exceptionally lucky retrovirus hit upon the subtle changes required for the effect. Likely? No, not at all. Combined with the excessive mortality, I'd say no way on this green Earth. (You may want to think about the mortality rate; it looks waaay too high to me.) Would I accept this setup in a novel or a film? Yes, of course. After all, it's not as if I was asked to believe that an interstellar ship can work as a submarine, or that Norse gods literally walk the Earth... $\endgroup$
    – AlexP
    Mar 27, 2019 at 23:00
  • $\begingroup$ @Agrajag Thx for the tip, will check later. The virus is transmitted through the air (updated question), but I can't say much about physiological effects or further classiffication, since I didn't think of it quite yet. Left it for later, because I felt it was more important to verify if such mutations were possible. Since the (now) obvious answer is yes, I'll provide you with some more specific questions very soon. Thx for the reply $\endgroup$
    – scipio
    Mar 27, 2019 at 23:56
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    $\begingroup$ @AlexP So, would it be more likely if the population infected was pretty big (to counter the high mortality rate)? Like say a continent or something? The idea is pretty much based on the Spanish colonization of the Americas. Edited: As for the "exceptionally lucky" part, will have to rely on it, for now. Thx for the feedback! $\endgroup$
    – scipio
    Mar 28, 2019 at 0:03
  • $\begingroup$ @Brythan It is naturally occurring, just updated the question. Yeah, I realized just how broad the question is. Mostly stems from my lacking understadning of viruses, their diversity etc. $\endgroup$
    – scipio
    Mar 28, 2019 at 0:08

5 Answers 5

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(1) Ebola

  • Changed one doctor's eye color from blue to green. Although in this specific case his eye color did change back, the article notes:

    Though it is quite rare for eye color to change so dramatically, this does happen from time to time as a result of viral infections and is usually permanent. Changes in color are usually due to the viral infection damaging pigmented cells in the iris. Following treatment, however, Crozier’s eye returned to normal, though it remains unclear why.

  • There are links between viral infections and graying hair color

  • Had a kill rate as high as 89% kill rate in the Democratic Republic of the Congo

  • Although Ebola doesn't pass on genetically, in a medieval-type world without proper sanitation (or even awareness of biotics) it could easily spread to children, especially if one or both parents had the disease.

  • If your story included a mutated form of Ebola that attached to people genetically, the other criteria make sense.

(2) Shingles

  • Shingles is a crazy virus (a form of herpes) that often lays dormant for many years
  • can cause severe hearing loss or blindness. In rare cases, can lead to death
  • again, by default it doesn't pass on genetically, but (a) is part of the same virus that causes chickenpox, which was considered "norm" for children due to its prevalence until vaccines were made for it. And (b) a mutated form of Shingles would not be difficult to image, one that attached (passively) to children during insemination, but laid dormant.
  • although mortality rates of shingles is low, a mutated form could be more deadly. Given its tendency to lay dormant without detection or activation for many years, it's unlikely a medieval-type society would be able to prevent it.
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    $\begingroup$ This answer deserves a bounty. $\endgroup$
    – The Square-Cube Law
    Mar 28, 2019 at 0:52
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There are lots of things that can depigment hair and skin. Some drugs can do it, but that is reversible. Age of course can do it. Vitiligo is an autoimmune condition; affected skin loses pigment and hair coming from that skin can be white. It is possible to develop vitiligo because you have melanoma - an immune attack on the pigmented tumor cells also attacks pigmented non tumor cells. I do not know (until just now) of any virus which leaves white scars.

Iris depigmentation is rare. Drugs that depigment hair and skin don't affect the iris, nor does age (not counting arcus senilis) Vitiligo does not usually affect the iris. But viral infection apparently can cause depigmentation of the iris.

Bilateral acute depigmentation of the iris: report of 26 new cases and four-year follow-up of two patients.

depigmented iris from HZV https://aibolita.com/eye-diseases/38504-gregory-ostrow-md.html

As I understand it this is not an immune attack on the pigment cells but direct infection and destruction of those cells by virus.

So yes, your virus could cause lasting pigment change.. Have the melanocytes be one of the targets of this virus such that they are destroyed throughout the body. Or have an immune attack on the virus that hits melanocytes. Or both.

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  • $\begingroup$ This isn't depigmenting. There is no blue pigment in humans. Blue eyes are blue for the same reason the sky is: refraction. So to cause a blue to green or gray requires adding pigmentation. $\endgroup$
    – Brythan
    Mar 28, 2019 at 0:13
  • $\begingroup$ @Brythan - does that eye look blue to you? Or are you commenting on the wrong answer? $\endgroup$
    – Willk
    Mar 28, 2019 at 0:31
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    $\begingroup$ From the question: " Eye colour has changed from blue to green; if not green then gray." If you're not turning blue eyes green or gray, then you're answering the wrong question. $\endgroup$
    – Brythan
    Mar 28, 2019 at 0:35
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    $\begingroup$ I thought the picture showed an eye which had (partially) turned gray. Gray eyes a person is born with are basically blue eyes as regards pigment. Eyes that turn gray as in this image do so because pigment is lost or dispersed. $\endgroup$
    – Willk
    Mar 28, 2019 at 2:10
  • $\begingroup$ @Brythan You're on point. Nevertheless, he provided some interesting insight that may be useful later on. $\endgroup$
    – scipio
    Mar 28, 2019 at 13:22
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Genetically transferred virus

User cegfault did a good job showing that Ebola would be a good candidate virus for killing 90% of the infected and causing grey hair and green eyes. But Ebola is not passed on genetically. BUT there are ways viruses could be passed on genetically. Here's how:

Retroviruses insert themselves into the DNA of our cells, but we don't pass on most cells to our children (e.g. chickenpox lives in nerve cells not in sperm/eggs). However, very rarely, viruses do get into our sex cells and so get passed on (when I mean rarely I mean back when our ancestors were lemurs). We know this has happened before and some scientists think it may be behind multiple sclerosis and schizophrenia (this is a super cool theory by the way, basically this virus can get activated during an illness and depending on how our body responds we can get MS or schizophrenia). Now its believed that these viruses accidentally ended up in that ancestor's sperm/egg cells but we could imagine a virus that preferentially inserted its DNA into sperm/eggs just like chickenpox hangs out in nerve cells.

Ok great, now we've got a virus in the parents sperm/egg DNA but still need to inherit that virus from both parents to have the change in hair and eye color. Well, if we inherit one "healthy" side of DNA that DNA continues to make the correct eye color, perhaps even compensating for the "bad" DNA (just like how carriers of albinism aren't lighter skinned but if they have children with another carrier 25% of the children will be albino). The only problem so far is that Ebola is not a retrovirus so cant insert itself into your cells (it only uses RNA not DNA). But perhaps a satellite virus could perform horizontal gene transfer between viruses and a retrovirus could obtain those attributes of Ebola that we are interested in. Voila, now you can get the virus from your parents or from the air (and once you get it from the air it inserts itself into your sex cells and you pass it on to your kids). Now why doesn't the virus from your mum's cells infect the cells with your dad's DNA? Perhaps the cells only insert their DNA into the sex cells.

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A genetically engineered retrovirus is fully capable of changing genetic traits, including eye and hair colour, but as your setting is medieval, I doubt anyone is engineering viruses.

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  • $\begingroup$ I updated the info, it's a naturally occurring virus. $\endgroup$
    – scipio
    Mar 28, 2019 at 0:21
  • $\begingroup$ Okay, that makes things harder. I doubt any viruses could evolve on their won to modify humans in such ways, but I'm not an expert. $\endgroup$
    – Ushumgallu
    Mar 28, 2019 at 2:51
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It is certainly possible for a virus to change hair or eye color. (See other answers for examples. I can also offer the example of a friend whose hair color changed permanently after chemotherapy.)

It is certainly possible for a virus to worm its way into our DNA and cause heritable mutations; a big chunk of our DNA is viral insertions.

What seems less plausible is a virus that causes inheritable changes to visible characteristics. Your virus needs to be pretty effective to hit body cells as well as germ cells.

Darwin's Radio, from maybe 20 years ago, explored the effects on society when people started giving birth to children who were... different.

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