Pregnant women are among those being urged to get the flu vaccine in a major public health drive.
People with existing health conditions and parents of young children are also being targeted by medics seeking to encourage widespread immunisation against the illness ahead of the winter.
Public Health England (PHE) said pregnant women are urged not to put off the free jab as pregnancy naturally weakens the body's immune system, increasing the risk of a mother and her unborn baby becoming seriously ill.
Older people, the very young and those with a health condition such as asthma, diabetes or heart disease, are also at "particular risk" from the most serious effects of flu, PHE said.
Since last year children aged two and three have been eligible for flu vaccination with a new nasal spray. It is now available to four-year-olds.
A national media campaign to alert at-risk groups is launched today, and will run for four weeks.
Chief Medical Officer Professor Dame Sally Davies said: "Flu is a really unpleasant illness, particularly for our most vulnerable patients and it is essential that people take steps to protect themselves during the winter months.
"I would urge those who are offered the free flu vaccination to visit their GP early in the flu season. I also urge all health care workers to make they are vaccinated to protect themselves, their patients and their families".
Dr Paul Cosford, director for health protection and medical director at PHE said the nasal spray for young children is "quick, easy and painless", adding: "T he vaccine also helps to reduce the spread of flu to those who are more vulnerable."
He went on: " "The best way people can protect themselves from flu is to take up the offer of free vaccination from their GP as soon as it becomes available.
"Even people whose health conditions are well managed and who lead otherwise healthy lives should still have the flu vaccine - it's free because you need it."
Last year around 40% of pregnant women were vaccinated, Dr Cosford said.
"This year we want to see more pregnant women and their babies protected," he added."Women can safely have the vaccine at any point during pregnancy and it can reduce the risk of complications such as pneumonia and premature birth, that can arise as a result of flu."
Each winter hundreds of thousands of people see their GP and tens of thousands are hospitalised because of flu.
Last winter, 904 people were admitted to intensive care or high dependency units with flu and, of those, 98 died.