We know that we wash our hands because germs can get into our body through the n

Question

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:

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

Explanation / Answer

There is a Wikipedia page which gives some typical values for infectious doses of bacteria. I suspect that lot of these have been taken from this slideshow which confirms the values on the Wikipedia page, gives more interesting details of how they have been arrived at, and also mentions a few viruses.

It seems from the slideshow that infectious dose isn't estimated in the way that the OP envisages - "What is the probability of infection if I ingest one cell?".

What is clear is that the infectious doses for some bacteria can be less than 10 cells (e.g. Shigella flexneri, E. coli O157:H7). Conversely for many other pathogens such as other strains of E. coli the estimate is >106 cells, so standard hygiene precautions will always offer some protection.

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