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Is there something in immunology that prevents a simultaneous infection with a 2nd pathogen? For example, I've never heard of someone getting both dengue and malaria together. Or, say, Ebola and Marburg fever. Of course, sheer probability makes it unlikely but is there something more fundamental that prevents it?

Maybe a more tractable version of the question is within the class of viruses or one type e.g. respiratory viruses. Is there something that prevents getting infected by multiple pathogens at once?

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You can absolutely have two infections occurring together.

One term which is used is 'co-infection'. Also 'secondary infection', in case where either the first infection or the treatment made it more likely for the second infection to occur. For example, secondary infections causing pneumonia were a big issue with Covid19.

I have done a quick google search on 'dengue malaria co-infection', and cases do exist. In this paper (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3614227/ - Concurrent malaria and dengue infection: a brief summary and comment), the authors comment on the fact that for this particular combination of diseases, the co-occurence is indeed quite low. They have various hypotheses:

  • The vectors for either disease are specific to geographically distinct areas, e.g. mosquitoes that prefer city vs rural areas. (I've seen it contradicted in another paper but no time to search).
  • Pure statistics: the likelihood of getting both diseases is much less likely than getting one, especially if getting one disease results in behaviour modification (e.g. staying at home sick).
  • Underdiagnosis: if you go to the doctor with one obvious disease, you probably will not get tested for other co-infections you might have.

It is possible that, for other pairs of diseases, immunity may play a role (your body is already on high alert from one infection) but that can also provide a risk (your body is so exhausted from fighting infection #1 that you can't fight infection #2). I am not aware, though, of any general statement that would apply to all diseases.

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    $\begingroup$ Another particularly good example would be the hepatitis D virus, which requires the hepatitis B virus to be present for it to replicate (though not all cases are coinfection, HBV carriers can also get HDV for example). $\endgroup$ Apr 28, 2023 at 2:10
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    $\begingroup$ Something from not too long ago - man diagnosed with Covid, Monkeypox, and HIV: webmd.com/covid/news/20220826/… $\endgroup$
    – Barrington
    Apr 28, 2023 at 4:30
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    $\begingroup$ Another related term is "superinfection". Especially common in immunocompromised populations and/or when antimicrobial therapy was inappropriate. $\endgroup$
    – M.A.R.
    Apr 29, 2023 at 17:16
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    $\begingroup$ I've got COVID and a bacterial chest infection as we speak... Feels a very relevant example $\endgroup$ Apr 29, 2023 at 22:08
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    $\begingroup$ Another factor factually making a secondary, subsequent infection less likely is that the immune system is all revved up already. The second pathogen encounters a really hostile environment. During an acute infection the body is in an exceptional state which it is not able to maintain in ordinary times (I guess because of detrimental side effects -- think fever --, auto-immune issues and resource issues). $\endgroup$ Apr 30, 2023 at 14:25
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There are viruses that require coinfection. The Hepatitis D virus is incomplete and can only reproduce in a host who is also infected with Hepatitis B.

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  • $\begingroup$ Very interesting point, although quite a special case. $\endgroup$ Apr 30, 2023 at 14:29

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