More than 8,400 children in Germany are suffering from a virus which causes severe nausea and vomiting, and officials say they think the common denominator is one type of food which was served at several different schools and daycare centers.

The number of affected children took a sudden jump from about 4,000 over the weekend, according to the Christian Post.

“Officials in Berlin, as well as the nearby eastern German states reported the further outbreak on Saturday. They announced that a huge number of new gastroenteritis cases had been reported to them, and that the cause was still being investigated.”

Of course, isolating the offending bacteria could be a hard job, and investigators say they are advising all well children to bring home-packed lunches until the mystery is solved.

The majority of cases have resolved themselves in a couple of days, and officials haven’t seen many hospitalizations as a direct result of the tainted food.