More than a dozen people have been hospitalised in Belfast after a substantial increase in the number of people infected by cryptosporidium.
Residents of west and south Belfast have been instructed to take hygiene precautions after 15 people were hospitalised. More than 50 cases of infection by the parasite have been reported in the last two months.
The parasite, which can cause vomiting, cramps and chronic diarrhoea, poses a particular risk to the elderly, very young and infirm.
It infected over 120 people in an outbreak last summer when water supplies in west Belfast were contaminated.
The North's Eastern Health Board said it was not yet necessary to boil water as the source of the infection might not be the water supply. The board said an investigation was under way, but it had established no link between the previous outbreak and the recent rise in cases.
"There is an expected seasonal increase in cases of cryptosporidium at this time of year. However, in the current year the increase is greater than usual," it said.
"The board therefore recommends good hygiene practice, especially hand-washing before eating and avoidance of handling or preparing food for others when ill." Dr Brian Morgan, a consultant with the board, said there were a number of possible sources of the outbreak: water supplies, food, contact with animals, human contact, travel abroad and swimming pools.
The steady rise in cases since February, which may not yet have peaked, indicated that a sudden contamination might not be the source, as was the case last year.
"A very detailed investigation is under way. But it is imperative that anyone suffering acute gastroenteritis should contact their GP right away, and it is essential that people follow good hygiene practices," Dr Morgan added.
Those most vulnerable to infection have been advised to boil all water.