An outbreak of the Norovirus, which causes diarrhoea and vomiting, has hit 15 schools, day care centres and care homes in Wiltshire.
The Health Protection Agency is looking into the cause of the outbreak, which has affected more than 50 people.
It advises people to take care with their personal hygiene, but says there is no specific treatment for the virus, "which usually clears up by itself."
One council resource centre and a school have been closed.
In the care homes affected, no new admissions are being allowed.
The virus lasts around 24 to 48 hours and only infects humans.
Exeter House Special School and Sarum Resource Centre, both in Salisbury, have been closed by the county council,