Complications from e.coli poisoning can be serious.
The number of confirmed cases of e.coli food poisoning in the south Wales valleys has risen to 31 in 16 schools.
As food suppliers were being examined as part of the inquiry, there were fears the total might increase further.
Most of those affected in Rhondda Cynon Taf and Merthyr are children, and at least 11 have had hospital treatment.
The number has risen from seven at the weekend. Schools opened as usual on Tuesday, but parents were told to keep children with a stomach upset home.
The National Public Health Service for Wales said the total went up to 22 on Monday. Another one was confirmed on Monday afternoon, and eight more on Tuesday.
A helpline has been set up with five lines, on 029 2040 2520, open from 0900 to 2100 BST every day until Friday.