People living in several parts of Galway city and county have been advised to boil their water before use following an increase in the number of people in the area becoming ill with diarrhoea.
Their illness, it is believed, is caused by ingesting a microscopic parasite called cryptosporidium.
And while drinking water is one of the possible routes of transmission of the illness, testing of the water conducted to date has failed to detect the presence of the parasite.
A joint statement issued last evening by the Health Service Executive (HSE) in the west, Galway County Council and Galway City Council, advised the public to boil water used for human consumption until further notice.
It said: "In the last month the public health department, HSE West, has been notified that there has been an increase in the number of people in the Galway area becoming ill with a diarrhoeal illness. The particular type of illness is caused by ingesting a parasite called cryptosporidium.
"Drinking water is one of the possible transmission routes, although testing conducted to date has failed to detect the presence of the parasite cryptosporidium.
"As a precautionary measure, HSE West has advised Galway County Council and Galway City Council to introduce a boiled water notice to all water users" in a number of areas.
"This measure is advised in an attempt to reduce new infections," it said.
The areas to which the notice applies include Galway city, Barna, Carnmore, Athenry, Claregalway, Corofin, Headford, Lackagh, Oranmore, Tuam and Turloughmore.
Those supplied by public water schemes in these areas and 80 group water schemes are advised to heed the notice.
The statement said water used for drinking, preparation of salads and similar foods which are not cooked prior to eating, brushing teeth and making ice should be boiled.
Domestic water filters will not render water safe to drink.
It says when preparing baby feeds, bottled mineral water is no substitute as most brands contain concentrations of minerals that are too high for babies.
Symptoms of cryptosporidiosis include diarrhoea, stomach cramps, upset stomach and a mild fever, and can last for two weeks. However, the parasite may continue to be in a person's system for up to two months.
The HSE West advises infected people to drink plenty of fluids to prevent dehydration. It also says anyone suffering from diarrhoea for more than two days should contact their GP and provide a stool sample for testing.