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If someone from our Earth- from, say, NYC or some other big city- ended up on Planet "Qwerty" for some reason or the other and bit someone in a no holds barred fight, would diseases transfer between the combatants? If so, which direction is more likely and how quickly would effects show?

Assuming the planet in question has superstorms that isolate most communities and neither the Earthling nor the "Qwert" are actually ill when the fight occurs. And that this is not the Earthling's first contact with the native species (like they've run into several people since arriving on the planet) but the Qwert has no idea that the Earthling is alien and has never seen them before.

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    $\begingroup$ You might want to check out my old question Would humans be able to derive nutrition from foodstuffs found on alien planets? It's not the same question, obviously, but a lot of the reasoning in the answers should apply to yours as well. $\endgroup$
    – user
    Apr 27 '18 at 7:03
  • $\begingroup$ Most organisms are covered with microbes. They can be transferred by contact. The probability that an alien microbe is adapted to either an Earthling or Qwert is extremely low. Acquiring someone else's microbes will not automatically lead to disease(s). A disease microbe needs to be adapted to a host's biology before it can be expressed as a disease. Very, very unlikely. $\endgroup$
    – a4android
    Apr 27 '18 at 7:11
  • $\begingroup$ If your guy just survives on another planet, you are in the realm of fantasy. Disease works as you please there. $\endgroup$
    – Raditz_35
    Apr 27 '18 at 7:19
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Human disease will only affect humans.

You might think a disease would spread rampantly among an alien population considering the effects Smallpox had on the south american natives when they were exposed to it. And while this is a perfectly reasonable assumption, it's also incorrect.

It is almost impossible for there to be any human diseases that can affect the alien at all. The reason for this is that human diseases evolved to infect humans.

Alien Cells would be too different from Human Cells for any disease to attempt to infect. For example, a Plant can't catch Smallpox from you.

And while there is some precedent for cross-species infections as has been the case on Earth, these have been between animals that share a relatively recent common ancestor, like Cats and Humans, or Birds and Humans. This is why I said "almost" impossible, because there is a tiny, tiny chance it is compatible, but this is less likely (and less plausible) as winning the Lottery every time for about 100 years.

But that doesn't mean that there's no way for you to make it plausible. Using something like the Panspermia theory, you could have the two species share a common ancestor, which would massively raise the odds of an infection.

War of the Worlds can be considered incredibly inaccurate. Until, that is, you consider the theory that the Martians and the Humans are biologically connected from a common ancestor, which makes it just slightly more likely, and into the realm of plausibility.

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    $\begingroup$ The fact that the guy is even able to survive on that planet tells me that the virtually impossible has happened and they have a compatible biochemistry and other implications like that. Arguing at this point about what is likely and what isn't is kind of like pointing out that in a story where a guy wins the lottery 200 times in a row, it's unlikely to the point of it being impossible that he wins it a 201st and 202nd time back to back $\endgroup$
    – Raditz_35
    Apr 27 '18 at 7:23
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    $\begingroup$ However, some viruses are actually able to infect creatures across-species. The fact that they can survive on the same planet under the same environmental conditions means that the species are not dissimiliar enough to have completely different metabolisms, both are in some way oxygen-based, they can share carbon+protein-based food and drinks in the same bar etc. $\endgroup$
    – Alex2006
    Apr 27 '18 at 10:07
  • $\begingroup$ This is a result of Convergent Evolution, in which two very different animals (For example Dolphins and Sharks) evolve similar traits despite a common ancestor being very distant for them, as one is a mammal and the other a fish. While outwardly the two species may look alike, on a deeper level their cells would be too different to contract disease from each other as they do not share a common ancestor. $\endgroup$
    – Sydney Sleeper
    Apr 27 '18 at 11:16
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    $\begingroup$ This is true for virus, which depend on receptors on cells for infection. But a bacteria may infect you just because you are a good environment for it. $\endgroup$
    – The Square-Cube Law
    Apr 27 '18 at 12:47
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What we call disease is a non normal interaction between (our) organism and another one.

If you consider that our skin and mouth host millions of bacteria, and likely will do your qwerties, it is easy to understand that chances of transmission are equal, and that the chance of getting a disease are also equally split.

But to get a disease you will need to have a bacteria or virus which can use the host organism to reproduce.

If you have read the War of the Worlds, you know how this can end..

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When we speak about infection, we are either thinking about microbes, virii or prions.

Virii will only infect aliens if those aliens have something akin to our cells in their phisiology. The virii would also require the aliens to have the same receptors in their cells' membranes in order for virii to work. So this relies heavily on aliens' biology being based on DNA, or at least RNA, which might not always be the case.

Diseased prions do their nasty work by converting sane prions into a bent shape. If the aliens don't have prions, these are out of the question as well.

When you get to bacteria and protozoa, though, it's a whole other story. Unlike prions they are not zombie protheins, and unlike virii they don't get into your body to steal your ribossomes. They hang inside you because you are a good environment for them to live and reproduce.

In fact, most bacteria within us don't cause any harm, and some are even necessary for our good health.

Now, imagine that you have cavities. Those are caused by bacteria eating through your teeth enamel. For us humans, specially after the 20th century, these bacteria are just a nuisance at worst.

But imagine that the alien you bit has a carapace made of the same material as your tooth enamel (but you happened to bite an unprotected, soft body part). If that alien doesn't get treatment, they may end up with serious dermal injuries over the course of a few weeks.

There are many species of bacteria that live in our surroundings, but don't thrive within our bodies - mostly because they can't handle the competition with the ones we do carry inside. If aliens came in contact with humans, and the aliens' bodies turn out to be a fostering environment for those bacteria, then the aliens will suffer infections very often. If the aliens have any equivalent to our bacteria in their world, then we can add "and vice-versa" to it: what is harmless for them might be dangerous for us.

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