The Department of Natural Resources is warning people that a condition called swimmer's itch is being seen at northern Iowa's Crystal Lake.
The condition usually is harmless but can cause welts and itching for several days.
The DNR says Tuesday that the condition has been noted at Crystal Lake in Hancock County, about 30 miles west of Mason City.
Swimmers itch is caused by a parasite that can burrow into human skin before dying.
The DNR advises avoiding swimming in areas with aquatic plants, which can contain snails that host the parasite. People also should dry off quickly after getting out of a lake.