The bug is affecting patients and staff at the Broomfield Hospital in Chelmsford.

Two other wards have just re-opened after being closed for two days last week because of the illness.

The virus, which causes diarrhoea and vomiting, has so far affected eight patients and 12 staff.

To help control the spread of the infection, hospital managers closed wards B11 and B12 last week for 48 hours.

Now ward B13 has also been affected by the gastro-enteritis bug.

A hospital spokesman said such an outbreak was normal for the time of year and there was no need for alarm.