It’s safe to go back in the water — or at least no more dangerous than usual — county and state officials said Friday.

But they still don’t know why at least 172 got sick after boating near Big Island in Lake Minnetonka over the July 4th weekend.

“We do not believe there is an ongoing heightened risk of disease on Lake Minnetonka,” said Dave Johnson, epidemiology manager for Hennepin County Public Health.

County officials have interviewed 225 people reporting illness and confirmed that 172 of them — those who were in the Big Island area over July 4th and experienced vomiting, diarrhea or both — were part of the outbreak. That number could grow.

The illness affected people of various ages from different boating groups and families, Johnson said. However, since the weekend it doesn’t seem to have spread, he said. Most of the victims were sick for a few days.

The county on Tuesday closed Excelsior Beach after water tests revealed a high E. coli count. Though Big Island is within view of the beach, officials said the beach closing and the holiday illness are not related.

Three beaches in Minneapolis, as well as one in Chaska and one in Roseville, closed this week with high E. coli counts, a relatively common occurrence in summer.

Officials said that people needn’t be more wary than usual about recreational activities elsewhere on Lake Minnetonka, because bacteria don’t travel far.

“Dilution is the solution to pollution,” said Trisha Robinson, supervisor of the waterborne diseases unit of the Minnesota Department of Health.

The county has tested a stool sample of one of the sickened boaters but hasn’t determined what caused the illness.

“We may not be able to determine the exact pathogen that caused people to get ill,” Robinson said.

Municipalities test for E. coli because it’s an “indicator organism,” suggesting the presence of high bacteria levels of various kinds that could make swimming risky. It’s not related to the E. coli infections from contaminated ground beef or lettuce, Robinson said.

With hot weather predicted this weekend, people will want to go swimming but may wonder whether the water is safe, Johnson said. His answer: “It’s as safe as it usually is.”

He repeated these precautions: Don’t go swimming or prepare food if you’re sick. Shower before swimming. And “don’t use the lake as a toilet.”