A second wave of swine flu could be on its way after cases in England rose 80% in a week, government figures have shown.
At least 66 schools suffered outbreaks of the H1N1 virus when children returned for a new academic year, pushing the number of estimated cases from 5,000 to 9,000.
Sir Liam Donaldson, the chief medical officer, said: "Parents of children with underlying illnesses should consult their doctor (if they have flu) and children with flu-like symptoms and whose condition deteriorates should consult their doctor because that might be an indication that they have serious complications of flu or have another serious illness of childhood."
Increasing numbers of young people with swine flu correlated with the upsurge in reported outbreaks, said Sir Liam, and he encouraged schools to enforce basic measures to lower the risk of spreading infection.
Throwing away used tissues, washing hands with soap and water and covering mouths and noses when sneezing or coughing would all make a difference.
"Research across a lot of children's diseases does show that that cuts the rate," he said.