The Lough Mask Regional Public Water Supply is said to be contaminated with the cryptosporidium parasite
Irish Water are currently investigating the contamination a major water supply in Mayo.
Boil water notices have been issued to more than 46,000 people across the county.
Irish Water and Mayo County Council issued the notice after discovering that the water supply served by the Lough Mask Regional Public Water Supply is contaminated with cryptosporidium.
The parasite can lead to diarrhoea, along with other symptoms such as abdominal cramps, nausea, vomiting and mild fever.
Areas affected by the boil water notice include Castlebar, Claremorris, Cong, Ballinrobe, Ballyhaunis, Kilmaine, Knock and Westport.
A number of group water schemes are also affected.
You can find full details on the Mayo County Council website.
Sean Corrigan, Regional Information Specialist from Irish Water, said: “Public health is our number one priority and while we investigate the source of this contamination it is imperative that people adhere to the boil water notice.
"We will be contacting priority customers supplied by this scheme who have self-declared to Irish Water directly by telephone. Irish Water is working closely with Mayo County Council to investigate all aspects of the scheme. A continuous sampling and testing programme has been put in place," he added.