People, stop! Masks DO work, just the other way around!

Immune system is pretty costly, it takes a lot of energy to keep up. If energy is needed for other systems, down he goes. All fight-and-flight reactions shut it down. Say, you need to run from sabretooth tiger--your organism directs all resources into escape; sneezing or diarrhea are, at that moment, not your, but tiger's future problem.

Go into suffocation and what happens? Right, autonomous nervous system recognizes you are in imminent danger. Does the immune system work now? Are you open to all nasty critters, instead?