A bug that causes sickness and diarrhoea has been found in the water supply in part of North Somerset.
Residents in and around the small town of Clevedon have been told by Bristol Water, to boil their tap water before use after Cryptosporidium, a microscopic parasite resistant to chlorine, was discovered in the supply.
The company said the notice is likely to be in place for 48 hours.
Cryptosporidium is a parasite that causes diarrhoea, nausea, vomiting and stomach pain if it is ingested.
People with weakened immune systems due to old age or ill health are most at risk, although those with healthy immune systems can recover by making sure they drink plenty of fluids.
The bug is transmitted by contact with water, soil, food or surfaces that have been contaminated by infected faeces containing the parasite. Infection can occur in both humans and animals.
The boil notice is affecting properties with postcode BS21," Bristol Water said in a statement. "One test result from the raw water quality samples taken at Clevedon treatment works detected Cryptosporidium. Therefore, the advice to boil the water is a precaution."
It added that the water was now being "supplied from a different treatment works" after the first one was "taken out of operation".
Bottled water was being supplied to vulnerable customers in the area, it said.
Clevedon School and Mary Elton Primary School were forced to close last week because of the bug.