Many diseases can be passed when the stool (or remnants thereof) of one host ends up in someone else's mouth. This is referred to as the fecal-oral route (or alternately, the oral-fecal route)
There are (usually) intermediate steps, sometimes many of them. One of the most common causes is that the feces comes into contact with water, which is improperly treated before consumption, or food which is improperly handled. Typically, fecal-oral route diseases are common in developing nations with poor sewage treatment practices.
Some sexual practices such as rimming can also spread disease by the fecal-oral route.
Some diseases that can be passed via the fecal-oral route: