About 250,000 people in Northamptonshire are being told to boil tap water for drinking after a bug was found to have contaminated supplies.
Routine tests by Anglian Water found cryptosporidium in a sample from supplies to Northampton, Daventry and surrounding villages at about 0100 BST.
The parasite causes stomach upsets and the most common symptom is diarrhoea.
Anglian Water is now conducting further tests to track down the source of the bug and then they will flush it out.
The original tests found the bug in a sample taken from Pitsford Water Treatment Works which serves the area in Northamptonshire.
Twenty schools in the area that rely on tap water for drinking fountains have now shut.
About 108,000 homes across 85 communities in Northamptonshire are affected by the contamination.
Cannot play media. Sorry, this media is not available in your territory.
Around 250,000 people have been affected
Anglian Water vans equipped with loudspeakers have been sent out to warn people about the problem.
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 some time yet.
"It could take weeks," he said. "We are working to get to the source of it.
"Once we have got to the source of it, we need to sample, and once we have done that, and once we are happy that the samples have come back, 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."
Northampton MPs Sally Keeble and Brian Binley have called for a full investigation by the Department for the Environment, Food and Rural Affairs (Defra) into the pollution of the town's water supply.
In a statement the MPs said: "It is appalling that the water supply for such a large and complex urban area could have become contaminated with cryptosporidium which is potentially a very serious problem for elderly people, young children and those who have health problems.
"We hope that Anglian Water will very quickly establish the extent of the pollution, and how to deal with it."
Some supermarkets in Daventry ran out of bottled water because of panic buying in the morning.
Mary Ellis, 57, from the Headlands, said: "I went to Tesco this morning and every trolley had bottled water in it."
The Health Protection Agency (HPA) said in a statement: "At present there are no confirmed cases of people who have been infected with cryptosporidium.
"The agency has written to local GPs and hospitals asking them to be vigilant for signs and symptoms of cryptosporidiosis."
Symptoms include diarrhoea, stomach pains, dehydration, weight loss and fever. The illness can be more serious in people with weak immune systems such as the elderly or very young.
Care homes, doctor's surgeries and hospitals in Northamptonshire are priorities for bottled water but some supermarkets are restricting sales.
Penny Howell, head teacher of Chiltern primary school in New Duston, Northampton, said she decided to close after Anglian Water said they were not able to supply the 200 bottles of water she needed for the 203 pupils until Thursday afternoon.
Northampton General Hospital said it was coping with the problem as its kitchens had the ability to boil water for cooking and tap supplies had been isolated.
Anglian Water advises customers with worries about their health to contact their GP or call NHS Direct on 0845 4647.