Contaminated frozen strawberries from China sold to commercial kitchens of three companies that made cafeteria food for schools and kindergartens in Germany, among them Sodexo, have been fingered as the source of norovirus which has sickened over 11,200 schoolkids.

When the strawberries were thawed to be used in desserts, they weren’t always fully heated, and norovirus, which is particularly resistant to cold and heat, survived. The different thawing processes used in various facilities could explain why infections occurred in some schools but not in others, though all used the strawberries from the same shipment.