Most waterborne disease-causing microorganisms come from human and animal fecal waste. A small drop of fecal matter can contain millions of microorganisms of many types, some of which are disease-causing pathogens. Exposure to the microbial pathogens found in sewage can lead to short-term and chronic illnesses.
The most common types of waterborne illnesses are short-term gastrointestinal infections that cause stomachaches and/or diarrhea. The elderly, children, pregnant women, and people with compromised immune systems are at greater risk of contracting chronic illnesses from sewage-contaminated water.
A survey by the Center for Disease Control reported over 4,000 documented illnesses from recreational waters in the U.S. in 2005-2006. However this number is assumed to be low because waterborne illnesses are notoriously underreported. People often associate the most common ailments, stomach and digestive system problems, with what they ate for lunch instead of contact with water. Still, reports of illness resulting from swimming are on the rise.