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Sorry, but if you are that germaphobic maybe you shouldn't be finning around in an unnatural environment on life support. If you need air, take it. If you don't trust your buddy's hygiene, find a new buddy. Don't mean to be disrespectful, but I think it's a silly question. You practice like it's real, so when it IS real, you can stay alive. JMHO
FYI, it's well known that sick people are "infectious" prior to the onset of symptoms. That being said, I don't think that a scientific study has investigated how readily a viral or bacterial respiratory infection can be transmitted to a diver via saliva on a regulator mouthpiece. My thought is that the mouthpiece will get a little ocean water rinse in-between diver use. That should decrease the chances of transmission somewhat (lowers infectious dose of the bug). Assuming that the "healthy" diver is not immunocompromised, I'd think that the risk of transmission would be very low.
Purge it an inch from your mouth and consider yourself drilled.
So to those of you doing frequent S drills how do you handle a situation where you simply don't want the primary that just came out of someone's mouth?
1) never mind it getting rinsed between their mouth and yours
2) never mind the open water contaminants that any alternate is exposed to