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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:

  1. 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?
  2. Do we know a certain trend based on the classification of the germ?
  3. How about the most commonly found germs (whichever they are)?
  4. How about some of the most feared, but not so common, virii known to man, like HIV, ebola, SARS?

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").

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Kind of related: biology.stackexchange.com/questions/5395/… –  nico Dec 8 '12 at 12:10
    
the answer is closer to this - most of the disease causing germs continue to be around us all the time. When conditions are healthy, the immune system and competition with other more benign organisms keep them from becoming a full blown disease. Soap helps them from reaching critical numbers and does not affect the immune response much, helping to keep them in check. Antibiotic chemicals do not because they create the conditions where antibiotic resistant strains appear. –  shigeta Dec 11 '12 at 5:03
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2 Answers

Washing your hands is an exercise in probability rather than absolute values. An important thing to keep in mind is that not only is it very hard to kill all the bacteria on your hands, it's also not something you actually want to do. Most of the bacteria we encounter day to day are either benign or beneficial to personal health. We know that gnotobiotic mice (sterile mice) exhibit impaired development, especially in the immune system. We want to come into moderate contact with bacteria.

Because it is a balancing act, I think hygiene needs to be reframed. For one things anti-septic soap should be reserved only for situations when it is absolutely necessary. Anti-septic soap has a tendency to apply a selective pressure that encourages propagation of resistant strains. Situations where it is worth using would include visiting an international transit center (airport), working in an Operating Room ward at a hospital, or going to a cafeteria during an epidemic. In other situations, regular plain soap and water should be good enough to rub the majority of cells off your hands so that passive immunity has a good chance of eliminating any threats before they get to vulnerable parts of the body. If you have the opportunity, paper towels are better for getting cells off your hands then blowers.

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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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I think that aside from the probability of illness if you ingest a few cells of bacteria XYZ, one important factor to consider is the probability of finding bacteria XYZ on your hands. While the first may probably be fairly high for, say, Yersinia pestis, usually the second should be very very low for that bacterium. –  nico Dec 9 '12 at 22:02
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