Cryptosporidium lodged in the lining of the intestine
Ultra-violet light is being used to make a water bug at a Northamptonshire treatment works harmless.
Anglian Water said the technology was a well-known cure for the cryptosporidium parasite affecting supplies to 250,000 of its customers in the county.
The firm is still advising residents of Daventry, Northampton and nearby villages to boil water for drinking and food preparation.
Schools which closed in the aftermath of the contamination have now reopened.
Experts are still trying to identify how the parasite, which causes severe stomach upsets, got into the supply.
The main focus of inquiry is the Pitsford water treatment works, which serves the main water reservoir in the part of Northamptonshire affected by the cryptosporidium contamination.
An Anglian Water spokeswoman said: "Scientists are working around the clock to find the source of the problem and are investigating, in minute detail, the Pitsford water treatment works."
Bottled water has been delivered to schools, hospitals and care homes
The spokeswoman said: "We have brought in three ultra violet units and this is a well known technology for rendering cryptosporidium harmless.
"These units are being installed today. Meanwhile our advice is still to boil and then cool any water to be used for drinking or food preparation.
"We are advising that the bottles delivered to school are taken home by pupils and filled with cooled water from the tap that has been boiled. If it is kept in the fridge overnight children will have water for the next day.
"But we are delivering about 37,500 bottles of water to schools, hospitals and care homes every working day as long as the situation lasts, although this will be less at weekends as schools are closed," she said.
Anglian Water has apologised for the inconvenience and has written to 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.
Anglian Water advises customers with worries about their health to contact their GP or call NHS Direct on 0845 4647.
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