But now some children in north-west schools have gone down with the illness, often known as "winter vomiting disease".
Three hundred of the 900 pupils at All Hallows School in Penwortham are currently off sick.
Among those who picked up the bug is 15-year-old Jenna Clorin-Wright from Middleton in Manchester.
She said: "You feel terrible really for about three days. All my friends have come down with it, and there is a lot off school."
Symptoms usually last for 24-36 hours, but there are hardly ever any long-term effects.