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I just found out how European colonisation of Africa was aided by the spread of diseases such as small pox and measles, which the the colonisers were relatively resistant to.

Of course, its goes both ways; Africa was called the whiteman's grave because of malaria and other tropical diseases.

More relevant to the topic, I also came across an article which denounces missionaries who spread the risk of diseases to isolated tribes.

https://www.theguardian.com/global-development/2020/mar/23/the-isolated-tribes-at-risk-of-illness-from-amazon-missionaries

Made me wonder the impact one person visiting a foreign land might bring (as contrasted with trade which brings more contract). I hope that will be answered here.

This will depend on were the person comes from: urban, rural; tropical, temperate; history, etc.

I'm more interested in what organisms might lie dormant in symptomless or immune carriers who might not even know they were carriers in the first place. And also how they even survive in an population resistant to them.

Incidentally, as a Malaysian and someone who rarely gets sick, I can't think of anything on me that might spead and wipe out an isolated tribe. Can you?

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    $\begingroup$ Every moment there are numerous battles going on inside your body, count yourself fortunate that your immune system is quite effective... ;D $\endgroup$
    – user6760
    Jun 10 '20 at 3:57
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    $\begingroup$ How much similar is your fantasy world to our own? In other words, how long ago is the last contact between them? The biology may be quite different to the point being completely incompatible (and the diseases irrelevant). We have A LOT in common with our own parasites. That's why they are able to use our bodies. $\endgroup$
    – fraxinus
    Jun 10 '20 at 12:30
  • $\begingroup$ Influenza, hepatitis, chickenpox, Herpes. $\endgroup$
    – John
    Jun 10 '20 at 13:58
  • $\begingroup$ witcher.fandom.com/wiki/Catriona_plague $\endgroup$
    – ggf31416
    Jun 10 '20 at 14:50
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    $\begingroup$ "As someone who rarely gets sick" means you don't have enough germs to have side effects dye to your white blood cells. It does NOT mean that you don't have germs. $\endgroup$
    – Mooing Duck
    Jun 10 '20 at 22:46
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There are a lot of viruses and bacterias we are immune to which we could transport without knowing. Eppstein-barr-virus is a good example. Roundabout 95% of adults have antibodies against it, so nearly everyone gets infected sooner or later in his live, most of them never showing symptoms. Especially by vaccination we made ourself nearly immune against a lot of very dangerous deseases we still could carry elsewhere. Another point is, as you mentioned a portal to a fantasy world: evolution there could have taken an absolutely different path, you don't know which bacteria living symbiotic in our body can be very dangerous to your fantasy lifeforms. Could be that bacteria like Escherichia coli, Staphylococcus aureus or Candida albicans are able to kill whole populations over there.

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  • $\begingroup$ I think the symbiotic microbe aspect of this answer really nails it. On a world unused to human contact, having a human breath on you could easily be just like getting bitten by a Komodo Dragon. $\endgroup$
    – Nosajimiki
    Jun 10 '20 at 19:04
  • $\begingroup$ @Nosajimiki-ReinstateMonica It's a lot more likely that we wouldn't be affected by bacteria/viruses in a world without humans (bar magic, etc), the same way we generally aren't affected by canine aids and dogs don't get the flu. There are exceptions, such as all mammals able to get rabies, or stuff can transmit with prolonged contact with animals, such as bird flu. $\endgroup$
    – Programmdude
    Jun 11 '20 at 4:27
  • $\begingroup$ @Programmdude This is absolutely true of viruses and prions which have to hijack specific DNA/RNA, but symbiotic microbes are bacteria and fungi. They generally have the capacity to metabolize a much more random assortment of bio-matter. Cross species resistance to bacteria and fungi are generally not because of the microbe's inability to consume the host, but the host body's immune system being designed to stop them. $\endgroup$
    – Nosajimiki
    Jun 11 '20 at 14:36
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Virtually every disease could be spread provided it was contagious. Very few have no cases where the disease can not be carried by a person who showed mild or no symptoms, because those diseases tend to have problems spreading. (Though this can be caused by the symptoms, even lethal ones, taking a long time to develop.)

The only condition would be that your fantasy world did not have it. However, if you have first contact through the portal, the only possible safety would be parallel evolution. For instance, rinderpest, a cattle disease, jumped to humans and became measles. (There is some evidence that it jumped more than once before becoming endemic.)

Edit: Malaria and yellow fever, being mosquito borne, could also be brought in if something brought mosquitoes. This would normally be something like a ship, though.

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Botulism comes to mind. It's a particularly terrible disease that almost no humans ever get (though there is a risk from spoiled food; this is why you shouldn't eat canned food from a bulging can), but dormant spores from it are common, especially in honey, where the extremely low-water conditions keep them dormant. (this is why you're told to never give honey to babies, by the way.) These spores are quickly killed by the human immune system in children and adults (but as mentioned before, not infants), but your fantasy people could very well have no immune response to it at all. As far as I know person-to-person transmission is uncommon if it exists at all; it's only really carried in foods, especially preserved foods like honey and canned goods (it's resistant to most sterilisation processes, and it's not enough of a problem in humans or any Earth animals to go to more extreme measures), but it's not too hard to imagine an explorer to another world brings a jar of honey or some tins of beans, one of which may contain botulinum spores.

An alternative is having some perfectly benign bacterium to humans turn out to be deadly to elves or whatever species your fantasy world is inhabited by. There are a ton of bacteria that live inside the human gastrointestinal tract, including in the mouth, where they could easily be spread by a human explorer kissing a particularly attractive orc that they fall for. (or by sharing a drink in a pub, but that's got less of a story behind it!).

Harmless microbes causing havoc when they jump to a different species is a well-known thing. Swine flu is a well-known recent case of this; it's harmless to pigs, but when a mutated strain of it made the jump to humans, it caused all sorts of problems. I believe it's generally thought that most diseases that run the risk of killing the person with the disease originated this way, as it doesn't make much sense evolutionarily for a parasite to actually kill its host if it can't survive outside of a host. But if a virus that evolved as a mild disease in pigs suddenly finds itself in a human, well, it doesn't know it's in a human now, so it still does the stuff that is mildly irritating but not harmful to pigs, which it turns out is very harmful to humans, and that's how you get swine flu.

I think COVID-19 is a case of this too, but I don't know what particular animal it made the jump from or even if that's actually true anyway.

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  • $\begingroup$ Hopefully this peer-reviewed article will help clear things up. ncbi.nlm.nih.gov/pmc/articles/PMC3448763 Of note (paraphrasing): "Diagnosis was confirmed by real-time PCR detection of botulinum neurotoxin genes in faecal specimens from the infant. Two samples of honey from the bottle fed to the infant were examined and both contained botulinum spores. Molecular typing ... showed the botulinum isolates from the honey were indistinguishable from the botulinum strain from the infant but different from botulinum isolated from other UK cases." $\endgroup$
    – wwarriner
    Jun 10 '20 at 17:41
  • $\begingroup$ I was curious about why adults don't have the same risk. This 1975 paper showed that bactericidal effects start around pH 2.5, and infant pHs were measured from 1.5 to 7 (ncbi.nlm.nih.gov/pmc/articles/PMC1413111/pdf/gut00610-0069.pdf). None of those below 2.0 had significant bacterial counts. According to the following two articles, young health folks are mean 1.7 fasting, elderly are mean 1.3 during a meal and 4.9 fasting. It stands to reason that age-increasing stomach acidity may play a role. $\endgroup$
    – wwarriner
    Jun 10 '20 at 17:57
  • $\begingroup$ Though with all that said, trace botulism may not pose a risk to the other-worlders via consumption if they have a low-enough stomach pH. $\endgroup$
    – wwarriner
    Jun 10 '20 at 17:59
  • $\begingroup$ @wwarriner, good job with finding the references. Add them in the answer to improve it ;) $\endgroup$
    – L.Dutch ♦
    Jun 10 '20 at 18:45
  • $\begingroup$ Thanks! It's part of my literal day job to support other scientists doing research. I am hesitant to modify the answer because the new information would conflict with or substantially change the first paragraph. I'll defer to @Hearth to decide if and how to integrate the sources. Meantime the comments can serve their purpose in providing supplementation of the answer as written. $\endgroup$
    – wwarriner
    Jun 10 '20 at 19:35
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Cancer?

Having your protagonist give chlamydia to persons he enounters in fantasy worlds would be realistic enough but not that exciting. You could turn to other diseases to make your fantasy exciting.

There are people who hold that cancer did not exist in the ancient world and is a disease of modern times. For some cancers that is a stretch because some with obvious physical changes (e.g. breast cancer) were described by ancient writers. But there are persistent reports of cancers because caused by infectious agents and some of these might not have been described. These infectious agents could cause epidemic cancers in unexposed populations.

Some cases of Hodgkin disease (a blood cancer) are definitely associated with EBV infection. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2360469/

There is controversial research associating glioblastoma with a different virus called CMV. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3922527/

There is no question that Kaposi's sarcoma is associated with a virus (KSHV; Kaposi sarcoma herpes virus) and also no question that the tendency to form cancers after virus infection has to do with a person's genetics. Africans and people of African stock can be immunologically normal and get "endemic" Kaposi's that is disfiguring and bad. For caucasians to get similar tumors they must be immunosuppressed.

If you want your character to be a plague dog with disease in his wake, these cancers and others could be the diseases. Cancer is dramatic and terrible and would work for a story. All the listed viruses are herpes viruses and many people are asymptomatic carriers of several. If cancers are driven or associated with viruses it is plausible that unexposed persons in a different land might have a higher rate of these cancers or reduced resistance to them.

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Yeast

This could seriously update food activities, and produce poisons (alcohol) that their bodies can't handle.

anything

Actually, almost ANY organism could have nasty effect. So called "flesh eating bacteria" is a common bacteria found on skin. Of course, their sensitivity to its toxins could be different, as could its ability to penetrate their skin.

Rabbits, in Australia, are another reasonable example.

and worse

An interesting related point. Antibiotics are actually toxins selected for toxicity to microbes we don't like and not to us. A transmigrator taking antibiotics or antibiotic cultures might find the antibiotics toxic to the other world residents ... and maybe not their microbes.

and even worse

The transmigrator himself might produce a toxin. For instance, suppose urea is unknown and a deadly toxin to them.

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Retroviruses have infected the human genome

Retroviruses have a rather unusual life cycle. Normal viruses inject their genetic material inside a cell, where it hijacks the cell's transcription machinery so that the cell begins following the directions that the virus genes are giving it. Retroviruses get into the nucleus and modify the DNA of the host cell, creating in effect a new variant of the original host DNA plus the retroviral genes reverse transcribed into it.

The human genome turns out to be littered with DNA of retroviral origin. I believe the working theories are the retrovirus successfully infected either sperm cells or egg cells and/or the testes tissue that makes sperm cells and/or an embryo in the early stages of gestation. The viral DNA would essentially be an integral part of the resulting child, and thus potentially all of that child's descendants.

It appears to be possible for these virus genes to be reactivated, meaning the human body will essentially infect itself with this ancient virus.

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