The number of cases of diarrhoea and vomiting has shown an increase in the last two to three weeks, health chiefs are reporting.
Most of these seem to be caused by a virus called the Norovirus, or winter vomiting virus.
The Public Health Directorate bosses say people infected should not go to work or school or visit public places, such as hospitals or care homes.
Anyone who has suffered diarrhoea and/or vomiting should stay away from work or school for 48 hours after symptoms end.
Everyone should practise good hygiene, washing hands thoroughly and cleaning all surfaces likely to have become contaminated.
Anyone who falls ill should drink lots to keep hydrated.
They should avoid going to the doctor’s surgery. If they need to contact your GP, the NHS says they should phone.
A Department of Health spokesman said: ‘It is important that all members of the public ensure that they play a part in preventing the spread of the virus.
‘Outbreaks of Norovirus infection has in the past resulted in closure of hospital wards, schools, care homes etc.’