The Norwalk virus has infected 120 people over the last three weeks.
It causes projectile vomiting and diarrhoea and has occurred mainly in town hotels.
The virus is easily passed on to guests and staff.
Environmental Health officer David Edwards said it was spread through airborne droplets or from contamination of surfaces and food.
Although not life-threatening, Mr Edwards said the illness could be very serious for the elderly and young children.
The virus was brought to the attention of Environmental Health as a result of reports from general practitioners and hotels about ten days ago.
