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Why is it rare for a person to have 2 (or more) infectious diseases (for example: Flu & Cold together at the same time)?

Although it's rare, it happens when the immune system is weak (e.g when patient has both AIDS and Kaposi's sarcoma).

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This needs a reference - I'd imagine it to be quite the opposite, probably we are often infected with many pathogens (even when not sick) and multiple pathogens would cause more stress on the immune system –  GriffinEvo Dec 24 '13 at 7:16
    
This happens more often than thought. Classic examples are secondary infections which get a problem, when the body is already battling one pathogen. This would be for example pneumonia or otitis after other infections like the flu. –  Chris Dec 24 '13 at 13:02
    
Your example is actually quite bad because the “common cold” is often an infection by several strains of viruses (and even bacteria) simultaneously. –  Konrad Rudolph Dec 24 '13 at 18:47
    
There are multiple issues here. 1) Is it really rare to be infected by multiple pathogens? 2) If it is rare, is it rarer than would be expected from the independent probabilities of getting infected by that organism within a given time period? 3) If both the above are true, I would speculate that could be because the body's non-specific immune mechanisms have already been activated by the first pathogen, making it harder for a new infection to take hold. –  Chinmay Kanchi Dec 25 '13 at 0:38
    
Because the product of two small probabilities is much smaller than either probability by itself? –  Superbest May 30 at 7:44
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What makes you consider it is rare? It is not at all uncommon to be affected by two infections. Typically I agree we aren't as the number of infectious organisms that we are exposed to which cause a clinical manifestation (i.e. aren't controlled by the immune system) are low. Nevertheless, any chronic infections such as herpes viruses may often manifest when we are under stress with another infection, many bacterial pneumonias are more likely to occur when a viral infection is also present and of course the many immunosuppressed people. Arguably a lot of infections are by two or more causative agents as often if one is able to enter where it shouldn't, another may, but one is likely to be more dominant. Furthermore even our infections can get infections with bacteriophages infecting the bacteria we are infected with.

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I like the "infections all the way down" comment. Can you add refs to bacteriophage infecting bacteria that colonize humans? –  PlaysDice May 30 at 13:51
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Secondary infection is not rare at all. I mean consider babies. When they get the cold their immune system is weak and focused on the cold virus and they can get viral pneumonia from the flu very easily for example.

TB is another good example of a disease often weakening the immune system and raising the probability of a secondary infection and HIV is a viral one with the same weakening and higher probability of secondary infection.

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