The number of people in the South West who have gone to their doctor complaining of stomach bugs has trebled in the past few months.
Norovirus - which is also known as the winter vomiting bug - is easily spread and outbreaks are difficult to control.
Symptoms include sudden onset of nausea followed by projectile vomiting and diarrhoea. Some people also get a high temperature, headaches and aching limbs.
People in hospital are particularly at risk. Many hospitals including Bristol's Royal Infirmary are restricting visiting hours. Four wards are currently affected at the RUH in Bath and Berrow ward is closed at Weston General.