E. coli is a bacteria that normally lives in human and animal intestines. Dangerous strains of E. coli can contaminate lakes and make swimming unsafe if sewage overflows into the water from rain or snowmelt. Signs posted by local officials warning not to swim should be heeded. E. coli infections may cause diarrhea, abdominal cramps, and other symptoms, and in rare cases can lead to serious blood or kidney problems requiring medical treatment. The bacteria primarily affects the urinary tract and kidneys. It is important to avoid swimming in closed bodies of water during an outbreak for safety.