Question

We know that we wash our hands because germs can get into ourbody through the nose, the mouth, the eyes, cuts, etc. But I doubtwe can completely clean our hands of germs every time we wash them,down to the individual bacterium or virion, even if we useanti-septic soap. So chances are, even if we keep up with ourhygiene, germs still get into our body, only in very very smallquantities. So here are some questions:

1. In theory, is there a dividing line where we can say "ok, atthis non-zero count of bacteria or virion, there is still (anarbitrary probability) that a normal healthy person won't getsick" for every germ known to man? If not, arethere germs that are known to cause havoc in a healthy person evenif he comes into contact with a single individual bacterium orvirion?
2. Do we know a certain trend based on the classification of thegerm?
3. How about the most commonly found germs (whichever theyare)?
4. How about some of the most feared, but not so common, viriiknown to man, like HIV, ebola, SARS?

I think that without being able to answer some of thesequestions, especially #3, hygiene would be nothing but a ritualactivity (because "it just works").