Water supplies contaminated with a parasite might not be safe for weeks, a water board has said.
Anglian Water told customers in parts of Northamptonshire to boil drinking water after cryptosporidium was found in samples.
The affected area, Northampton, Daventry and west of Daventry, is thought to cover 108,000 properties and 250,000 people.
Cryptosporidium is naturally occurring but can cause severe stomach upsets especially in young or elderly people.
Supermarkets were taking extra deliveries of bottled water to stores as people rushed to stock up on safe drinking water.
At Tesco supermarket in Daventry, shoppers were queuing in the bottled water aisle and loading their trolleys.
About 15 schools in Northamptonshire were closed because of the outbreak.
Dan Baker, spokesman for Anglian Water, said the investigation being carried out was a “big job” and might mean people having to boil their drinking water for weeks.
“We are working to get to the source of it,” he said. “Once we have got to the source of it, we need to sample, and once we have done that we need to flush the whole system to make sure there is no water left in there which was affected. So it is a really, really big job.”