My daughter is 8 months old. She puts her hand in her mouth a lot. Afraid of germs, my wife is trying to avoid this by removing her hands from her mouth. 2 minutes later she puts it again in her mouth! This is very exhausting for us, because we have to keep an eye on her 24 h/day. How to solve this?
People haven't been aware of germs until only recently (what's a 100 years in the entire history of human race?). With all the medicine available to cure most common sicknesses you should encourage your child to familiarize its immune system with common bacteria.
The general idea behind this approach is related to the Hygiene hypothesis (lots of information in this link). For example, you trade more diarrheas at early age for better health later in life.
Eating feces or dangerous chemicals shouldn't happen, of course, but absorbing some home dust, some sand and dirt will, in the long term, make your child healthier and more resistant to disease.
In other words, be reasonable. Some dust, some sand, some germs are perfectly OK. Boil the pacifier once a week, not twice per day. Wash hands after coming back from outside and before meals, and not every hour.
Consider this: Since children are still putting their hands in their mouths, after hundreds of thousands of years of evolution, isn't it most likely the "good" thing to do?
Also, remember that newborns and infants discover the world largely with their mouth. As much as it seems unintuitive for us adults, that's the case. By removing their hands (and other things) from their mouth you actually reduce the amount of stimuli they experience.