Scientists are testing water sources after fears that water supplies to 250,000 in Northamptonshire could be contaminated with a sickness bug.
Anglian Water scientists first discovered the cryptosporidium at 0100 BST during a routine test of Pitsford Water Treatment Works.
The works serves Northampton, Daventry and nearby villages.
Sara Rowland, of Anglian Water, said the firm routinely tests more than 1,000 samples a day.
Scientists at Anglian Water's laboratories in Huntingdon found the sample from Pitsford was "cloudy" and further tests revealed cryptosporidium.
Ms Rowland said extra staff were being brought in to the laboratories to carry out tests to determine if the bug is in the water supplies and what the source of the bug is.
She said Anglian Water staff were now getting samples from Pitsford and from sample mains in the area.
They are also taking random samples from taps in the Northampton/Daventry area.
Ms Rowland said it was still early days with the investigation and restrictions could continue for two weeks or even longer.
She said if it is found to be in the water system, it will then be flushed out of the system.
"It is not something which can be rushed in any way whatsoever - when we are talking about the public water supply. At the end of the day, our responsibility lies with the customer," she said.
She said as long as water is boiled, it is safe to drink.
Anglian Water is providing bottled water for schools and hospitals.
A spokeswoman for the Health and Protection Agency said at present there were no cases of cryptosporidium infection.
She said cryptosporidium is quite a common bug which can be be caught from faeces, for example when changing nappies, from infected pets or by swimming in, or drinking contaminated water.
You can also be infected by eating and drinking contaminated food, particularly unpasteurised milk, offal or undercooked meat.