Medical scientists in Co Galway are still trying to identify the cause of a serious outbreak of stomach illness in the county.
Laboratory tests have so failed to find the cause of the stomach cramps and diarrhoea and people are being advised to boil water before drinking it or using it for cooking.
Experts are trying to establish if water supplies to over 90,000 people have been contaminated by a disease-carrying parasite.
Local hotels, pubs and restaurants say they are concerned at the impact the alert may have on the tourist industry over the St Patrick's weekend.
Environmental health officers have visited over 1,000 pubs, hotels, restaurants and food outlets to take water samples.
The drinking water alert was issued after public health officials in the county became concerned at a steep rise in the number of people suffering from a stomach bug illness in recent weeks.
The diarrhoea-type illness is known as Cryptosporidiosis. Last year there were 74 cases in the three counties of Galway, Mayo and Roscommon. So far this year, Co Galway has had 43 confirmed cases.
Tests have not shown any traces of the cryptosporidium parasite in public water supplies, but as a precautionary measure all householders in the city as well as those in an area between Moycullen, Athenry and Tuam are being asked to boil water before using it.
The Health Service Executive's Director of Public Health in the west, Dr Diarmuid O'Donovan, says babies, young children, the elderly and those with weakened immune systems are most vulnerable to the illness.
At the moment, there are no plans to provide alternative water supplies.
