At least 66 schools in England have suffered swine flu outbreaks since the start of the school term, figures showed today.
New data from the Government also showed the number of swine flu cases across the country has almost doubled in a week, from an estimated 5,000 to 9,000.
Chief Medical Officer Sir Liam Donaldson said the number of cases was still relatively low, but there were signs a second wave of swine flu could be on its way.
He said the data on schools was not being systematically collected and he expected the number affected to be higher.
Asked if the figure of 66 was on the low side, he said: "I think there will be more than that."
He said the outbreak correlated with rises in the number of cases seen in young people.
Measures that would make a big difference in schools included regularly washing hands with soap and water, throwing tissues away and covering the mouth and nose when coughing and sneezing.
"Research across a lot of children's diseases does show that that cuts the rate," he 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."
Sir Liam said the National Pandemic Flu Service for England had seen a rise in the number of people seeking help for flu-like illness.
The last three days had seen figures "substantially above" the last few weeks.
Yorkshire and the Humber has seen 27 outbreaks of swine flu in schools since the start of term, which is the highest in England.
Sir Liam said some of this could be down to good reporting of outbreaks in the area but there had definitely been a "snowball effect" of cases in the region.
South Yorkshire had quite a few schools affected, he added.
Across Yorkshire and the Humber, just two schools had reported outbreaks last Thursday but the figure was up to 27 by yesterday.
Another 12 schools have been affected in the West Midlands, eight in the East Midlands, six in London, five each in the North West and North East, two in the south central region and one on the south east coast.
Across England, the number of people being treated in hospital has risen by more than 50%, from 143 last week to 218 this week.
Of these, 25 patients are in intensive care.
Across the UK, 82 deaths have been linked to the virus, with 70 in England (up three in the last week), nine in Scotland and two in Northern Ireland.
The Government is waiting for European regulators to licence the swine flu vaccine before it can be given to members of the public.
Those in high-risk groups, such as people with asthma or diabetes, will be vaccinated first, together with health workers.
Evidence suggests that people may only need one dose of the jab rather than two, although children may still need two after showing a lower immune response in clinical trials.
Sir Liam said he was "optimistic" that the country may be able to rely on one dose for adults.
Across Europe, France is being particularly hard hit by the current swine flu outbreak, today's figures showed.
While today's figures represent a rise in recent weeks, they fall far short of the peak of the pandemic in July when England was seeing around 100,000 cases a week.
The levels also fall short of the amount of normal seasonal flu that can be expected in winter months, although not typically at this time of year.
Sir Liam said: "I would have preferred not to have seen any increases yet and had more flatlining because that gets us close to the vaccine becoming available. I would have preferred to have had more breathing space."