Doctors in parts of south Devon have been asked to be on the look-out for signs of sickness after untreated river water got into the supply system.
The Health Protection Agency asked local GPs to be vigilant, although it says the risk to people is small.
Up to 16,000 households and businesses in the affected area have been told to boil water before using it.
South West Water is carrying out an investigation to find out how the water supply became contaminated.
AFFECTED AREAS
It is believed water from a stream infiltrated a water treatment works at Houndall on Thursday night.
Water in the affected areas can be used for washing, bathing and flushing toilets but residents are being asked to boil water for drinking, cleaning teeth, washing food, cooking and medical purposes.
A spokesman for South West Water said: "We apologise for the inconvenience this will cause and we are working to rectify the situation as soon as possible."
Plymstock school, which is in the affected area, decided to close on Friday and pupils were sent home. Some other schools bought in bottled water.
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