A senior infection control nurse has appealed for anyone who has had diarrhoea or vomiting within the last 48 hours to stay away from hospital.
Two care of the elderly wards and one bay on a contingency ward at Colchester General Hospital have been closed to new admissions because of norovirus.
The hospital says there are about a dozen patients with symptoms.
If you or anyone in your family has symptoms the advice from the hospital is to stay at home, ensure you drink plenty of fluids and remember to wash your hands before preparing food, before eating and after you have used the toilet.
If symptoms persist for more than a few days, call your GP or 111 for help and advice.