Coliform bacteria are common in the environment and are generally not harmful.  However, the presence of these bacteria in drinking water is usually a result of a problem with the treatment system or the pipes which distribute water, and indicates that the water my be contaminated with germs that can cause disease.

Fecal Coliform and E Coli are bacteria whose presence indicates that the water is contaminated with human or animal wastes.  Microbes in these wastes can cause short-term health effects such as diarrhea, cramps, nausea, headaches, and other symptoms.  They can also kill those with weakened immune systems.

Turbidity has no health effects in and of itself.  However, it can interfere with disinfection and provide a medium for microbial growth.  Turbidity may further indicate the presence of disease causing organisms.  These organisms include bacteria, viruses, and parasites that can cause symptoms such as nausea, cramps, diarrhea, and associated headaches.

Cryptosporidium is a parasite that enters lakes and rivers through sewage and animal waste run-off.   It causes cryptosporidiosis, a mild gastrointestinal disease.  However, the disease can be severe or fatal in people with severly weakened immune systems.  EPA and CDC have prepared advice for those with severely compromised immune systems who are concerned about Cryptosporidium.

Giardia lamblia is another parasite that enters lakes and rivers through sewage and animal waste.  It too causes gastrointestinal illness (e.g. diarrhea, vomiting, and cramps).