Fourteen schools in Northamptonshire have been shut because their tap water has been infected by a bug.
Water in the schools, and more than 100,000 homes, isn’t safe to drink without being boiled first because of the bug, called cryptosporidium.
The bug causes stomach upsets, and the schools are shut while experts try to find out how it got in the water.
Vans equipped with loudspeakers have been sent out across the north of Northamptonshire to warn people.
Although water in the area should be boiled if it’s going to be used for drinking, preparing food or cleaning your teeth, it is safe to wash in.