The flu season has begun, parents have been warned, after an increase in the number of children aged five to 14 falling ill.
Anti-viral drugs should be administered for people at risk of developing complications from flu, and it is not too late to have a vaccination, doctors advised.
Dr Richard Pebody, head of seasonal flu surveillance at the Health Protection Agency (HPA) said: "We are seeing an increase in flu activity mainly among school children indicating the start of this year's flu season.
"Flu vaccination is still the most effective way of preventing flu and it is not too late to get it so we would encourage all those who are in 'at risk' groups to get vaccinated as they are more vulnerable to developing complications from flu.
"These include people with underlying conditions such as heart problems, diabetes, lung, liver or renal diseases and those with weakened immune systems, as well as older people and pregnant women."
Flu symptoms include sudden onset of fever, cough as well as sore throat and aching muscles and joints.
The best treatment for otherwise healthy people is to stay at home and rest, drink plenty of fluids and take pain relief.
Under-16s should not take medicine containing aspirin.
Dr Pebody said: "Every season we remain vigilant and assess the flu situation as more information becomes available from our various surveillance systems and from the different virus samples we receive from across the UK."
Reports of flu are highest in North East England, according to a London School of Hygiene and Tropical Medicine survey.
Dr Alma Adler, Research Fellow at the London School of Hygiene & Tropical Medicine, who is running the UK Flusurvey project, said: "The data are very preliminary and flu levels are still very low but overall the results suggest that the English region most affected by flu is the North East with 19,200 per 100,000.
"We now need more people to sign up and let us know how they are feeling so we can study these figures in greater depth and increase our understanding of seasonal flu.
"It is also important to point out that it's not too late for those in at risk groups to get vaccinated."