Why do you have to wash your hands after eating?

A huge number of dangerous things, bacterial, viral and environmental, are spread by dirty hands. Note that you likely wipe yourself when you go to the bathroom, so your hands can carry a very large bacterial load. If you don't wash it off, you'll injest it yourself and spread it to others. Likewise, if you work or play around dangerous environmental pathogens, the same thing happens. Hospitals feel that the most important factor in controlling the spread of in-hospital disease is personal hygene. Some sources report up to 33% post operative infection in hospital, much of it non-aerobic. So hey! Lave los manos! :}