Problem:

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

In theory, is there a dividing line where we can say "ok, at this non-zero count of bacteria or virion, there is still (an arbitrary probability) that a normal healthy person won't get sick" for every germ known to man? If not, are there germs that are known to cause havoc in a healthy person even if he comes into contact with a single individual bacterium or virion?

Required:

Question 1: Do we know a certain trend based on the classification of the germ?

Question 2: How about the most commonly found germs (whichever they are)?

Question 3: How about some of the most feared, but not so common, virii known to man, like HIV, ebola, SARS?

Please answer the question.