Three swimmers contracted E. coli infections from water near Big Island in Lake Minnetonka during the July 4th weekend, prompting state health officials to issue summer health precautions for ­Minnesotans who swim outdoors.

One of the three swimmers was hospitalized, but all three have since recovered, the state Health Department said.

All beaches on Lake Minnetonka remain open and have passed regular water-quality tests, official said.

The incident marked the first case of a waterborne infection this year, but state health officials say more are likely to come.

Minnesota reported a record number of waterborne illnesses last summer, with 11 cases, but this is only the fourth E.coli ­outbreak since 2001.

E. coli, a bacteria that normally lives in the small intestines of humans and animals, can cause diarrhea and stomach cramps, which can be severe in some cases. Even a small amount of human or animal waste can introduce the bacteria to a swimming area, and ingesting even a small amount of tainted water can produce an infection. People typically fall ill within two to five days of swimming in contaminated water.

A man enjoyed the sun as he floated near Big Island in Lake Minnetonka on Friday. Three cases of E. coli have been found in swimmers in that area of the lake, and the state Health Department has issued a seasonal warning.

Health Department officials said an adult typically swallows about a tablespoon of water while swimming in a lake — enough to transmit E. coli if the water is contaminated — and children swallow much more.

The source of the bacteria in Lake Minnetonka is unknown, but lakes can be contaminated through animal droppings, sick swimmers, improper ­disposal of boat waste and other sources.

"This outbreak is certainly unique," said Trisha Robinson, a state epidemiologist who specializes in waterborne diseases. "E. coli is certainly an opportunity for severe illness. It's not something people like to think about, but we have to keep it in mind," said Robinson.

Lake Minnetonka's 125 miles of shoreline have remained open, because the incident occurred in a part of the lake that is considered a "non-beach area," according to Hennepin County Public Health spokesman Bill Belknap, who echoed the Health Department's health warnings. The three who became sick were swimming off boats anchored near Big Island.

Health officials said people should not be afraid to swim outdoors, but urged swimmers to take the following ­precautions:

• Don't swallow lake water.

• Shower with soap before and after swimming.

• Don't swim if you have diarrhea.

• Wash your hands thoroughly after you swim.

• If swimming with children, make sure they have frequent bathroom breaks; change ­diapers in a bathroom, not on the beach.

Ashley Griffin • 612-673-4652