Around 70,000 households across Gwynedd and Anglesey have been told to boil all drinking water following an outbreak of illness caused by cryptosporidium.
So far there have been 123 laboratory confirmed cases of Cryptosporidium in the Gwynedd and Anglesey area, with further cases expected, said the National Public Health Service for Wales.
Investigations remain centred on the Llwyn Cwellyn reservoir. Public health officials are testing to see if human sewage has entered the water course and DNA tests are being carried out to determine the exact strain of the cryptosporidiosis bug found.
All customers who use water from the Cwellyn Reservoir have been contacted by Welsh Water to advise them of the boiling notice.
People have been told to boil, then cool, all water used for drinking, food preparation, tooth brushing, ice making and baby feed preparations.
Welsh Water is continuing to work with local health authorities, local councils, Environment Agency Wales and the Drinking Water Inspectorate to discover the source of the outbreak.
Cryptosporidiosis can cause stomach cramps and diarrhoeal illness but is largely self-limiting in most people.
Anybody concerned about their health should contact NHS helpline on 08458 509 850.