Many diseases can be passed when the stool (or remnants thereof) of one host ends up in the 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 comsumption. This means that typically, fecal-oral route diseases are common in developing nations with poor sewage treatment policies.
Some diseases that can be passed via the fecal-oral route: