Health officials in southwest Idaho say norovirus is what sickened dozens of people at Eagle Island State Park last weekend.
The Central District Health Department says the likely source is a sick person who swam in the water or became sick at the park.
Department spokeswoman Kimberly Link says the virus can spread from one person to another through recreational water, food and direct contact.
She says human waste and vomit are the main sources of norovirus.
The highly infectious virus causes severe vomiting and diarrhea, but victims typically get better in a couple of days.
Park officials say the park will be closed for about two weeks so the water can be drained and the area cleaned.