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In working on the merpeople for my story - very basic, classic merpeople - they are humans underwater with fish tails. I've questioned whether they would contract diseases. It seems that they would in fact be able to, at least ocean based ones, like parasites, certain kinds of bacterial ones, etc.

The question I have though is if they would be able to contract ones like malaria, cholera, etc, from being in contact with humans. This is meant to be any kind of contact, but that of a closer variety, ie, kissing and sexually transmitted is less common. Contact in general is less common.

I feel that this is perhaps one of those "do what you want, they're basically humans" questions, and I could go that route, but I want this looked at basically through the lens of the aquatic side of merpeople. I'm also interested to know if exchange could happen vice versa - I know that many diseases result from livestock or wild animals, but I haven't heard of an aquatic borne disease - other than brain eating amoebas.

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    $\begingroup$ Can you tell us a bit more about the contact they have with humans? Do they eat/drink human stuff in the process, do they breath air? (I'm not familiar with "classic merpeople", unless you're referring to Scottish/Irish mythology, or some version of sirens from Greek myth). $\endgroup$
    – Escaped dental patient.
    Commented Oct 9, 2021 at 21:26
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    $\begingroup$ this entirely depends on their biology which we don't know. "merepeople" does not tell us anything about their anatomy. $\endgroup$
    – John
    Commented Oct 10, 2021 at 1:07
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    $\begingroup$ @John - edited. Humans with a tail. I'm sorry, I assumed classic merpeople was self explanatory. $\endgroup$
    – WasatchWind
    Commented Oct 10, 2021 at 3:10

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very basic, classic merpeople - they are humans underwater with fish tails ... The question I have though is if they would be able to contract ones like malaria, cholera, etc, from being in contact with humans.

Yes, but you'll need to investigate them one by one, based on the infection vector. For example:

  • Malaria has mosquitos as a vector, not going to be passed by direct contact with an infected person. But merpeople living around swamps or delta may definitely get bitten by an anopheles carrying malaria if surfacing for some time.
  • cholera - yes, Vibrio cholerae can be also found in brackish and coastal waters. Were I to be a merman, I'd be worried, since it's a water borne disease.
  • tuberculosis - airborne bacteria, can jump from person to merperson (or viceversa) if they sneeze one in the face of the other - or meet for a private dinner in close quarters under a diving bell.
  • plague - very unlikely to pass to merfolks, as Yersinia pestis uses flees as a vector - not a very good swimmer insect, even less a diver.

I'm also interested to know if exchange could happen vice versa - I know that many diseases result from livestock or wild animals, but I haven't heard of an aquatic borne disease - other than brain eating amoebas.

Surprisingly (for me at least), there are a number of infectious diseases affecting marine mammals (the closest approximation to merpeople) with known to make the jump on humans, enough of them to worry some researchers about zoonotic transmission to humans.
Brucelosis and leptospirosis would concern me, as I know they are common in zoonoses by land based animals (I'd need to read more about the others)

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We're helping you develop a rule of your world: and you have two options

  1. Merepeople are an evolutionary divergence from humans. Therefore, just as humans are similar enough to some primates that disease can move back and forth between them.1, 2

  2. Merepeople may look suspiciously like humans, but they're actually an example of parallel evolution. In this case, despite sharing characteristics with humans, they're not actually human or even primates. As a result, they're much better protected by the infamous blood-brain barrier and far, far less likely to become ill due to contact with humans.

Now, having said this, nothing's perfect. If I remember my biology correctly, nearly all viruses that afflict humanity began in birds. But very, very few of the viruses that affect birds end up affecting humans. This is because the illness must jump that nasty blood-brain barrier and almost always does so through a series of intermediary species (like pigs... e.g., swine flu) that allow the virus to mutate to eventually attack us.

If you follow #2, your merfolk have the same basic protection that humans have from all the really nasty illnesses that afflict the animal world. In this case, it would take a lot of contact with humanity (and the creatures humans keep with them) to eventually suffer an illness.

Conclusion

I believe those are your two options. If they're human mutants (or humans are merfolk mutants...), then disease would be reasonably easy and only a little contact with humans would lead to disease. If they're actually another species, then it would take a whomping lot of contact with humans to contract a human disease. But in either case, given enough contact, eventually somebody's going to get sick.

But, before I jump away, let's talk about saline

You could concern yourself with one thing: salt in the water. Illness (e.g., bacterium, viruses, etc.) like to feed on, well... food. Sugar is food. Salt isn't. It takes very little salt to kill something. Don't believe me? Drink a gallon of sea water in a day and see how you feel.

Your merefolk live in an environment that is naturally condusive to not being human-sick. This is an advantage — but it's only an advantage. Mother Nature tends to like throwing curve balls. Therefore, it is my belief that it's impossible for merefolk to be immune to human illness.

But I think it's unlikely they'd ever contract a human illness. It would take a lot of contact, less for #1 than for #2, but a lot nonetheless.

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  • $\begingroup$ This thing about salt is about the outside. Viruses live inside. Suppose infected humans on land come into contact with merfolk.. Greeting like friends, have dinner on the beach. Hugging, kissing when saying goodbye.. the usual close contact. How are you so sure the merfolk would be immune to any transfer of disease ? They are human bloodline, as you state.. inside.. they are human. As vulnerable as we are for micro-organisms that may affect us. From merfolk to humans on land, disease could be spread as well.. $\endgroup$
    – Goodies
    Commented Oct 10, 2021 at 3:10
  • $\begingroup$ ... (followup) voted the answer up. Reading the first part more careful, I think we agree, so please take my sentence with the question mark as a misunderstanding.. I can add there would even be increased risk, because of the isolated living environments. New viruses can be introduced, with little effective immune response, in both directions ! Indigenous people from all over the world shared this experience, when meeting their European colonizers.. $\endgroup$
    – Goodies
    Commented Oct 10, 2021 at 3:22
  • $\begingroup$ "far less likely to become ill due to contact with humans...it would take a lot of contact with humanity (and the creatures humans keep with them) to eventually suffer an illness." - yet is not impossible. We are not yet over with a pandemic caused by a virus jumping from bats to humans. Recent case of Hendra virus between species that come seldom come in direct contact. $\endgroup$
    – Adrian Colomitchi
    Commented Oct 10, 2021 at 3:45
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    $\begingroup$ "It takes very little salt to kill something. Don't believe me? Drink a gallon of sea water in a day and see how you feel." that gallon of seawater will contain about 0.94e12 viruses («A teaspoon of seawater typically contains about fifty million viruses.») $\endgroup$
    – Adrian Colomitchi
    Commented Oct 10, 2021 at 10:04
  • $\begingroup$ @Goodies I never said the merefolk would be immune. I simply proposed there's a difference in how susceptible they would be. I introduced the issue of salt because, living in the ocean and breathing underwater, their bodies would be more tolerant to salt and capable of processing it - but that doesn't change the fact that a salt water "atmosphere" is much less amenable to illness than our own saltless atmosphere. $\endgroup$
    – JBH
    Commented Oct 10, 2021 at 18:56

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