The government has launched a campaign to maximise the number of over-65s and children with health problems who are vaccinated against flu.
The vaccine gives 12 months protection
The jab is free to vulnerable groups, such as the elderly and children over six months who have asthma, diabetes or a weakened immune system.
Last year 30% of over-65s and half of children at increased risk did not have a jab.
The vaccination is available from your local GP.
Flu is highly infectious. A sneeze can carry the flu virus and, travelling at 80 miles per hour, reach distances of up to 30 feet away.
Flu is estimated to kill several thousand people in the UK each year
10-15% of the population develop flu each year
100,000 flu particles can be projected into the air with just one sneeze
In 12 hours, the flu virus can invade 1 million nose and throat cells
Touching objects such as toys that have been coughed or sneezed on can also pass on flu.
Sir Liam Donaldson, the Chief Medical Officer for England, said: "There is a big difference between a cold and flu - thousands of deaths are linked to flu every year.
"Flu tends to last for up to two weeks, giving you fevers, chills, extreme fatigue and aching muscles as well as the more cold-like symptoms of sore throats and runny noses.
"If you are 65 years or older, or you suffer from illnesses such as asthma, diabetes or serious heart or kidney disease you are particularly at risk from flu.
"For those most at risk, flu can lead to more serious illnesses, including bronchitis or pneumonia and could result in hospitalisation or, in the most serious cases, death."
Sir Liam stressed that at-risk groups needed to get a flu jab every year, as the flu virus is constantly changing.
Alpa Chhipa, whose asthmatic son contracted flu when he was two, said: "My son Miraj developed flu which brought on an uncontrollable asthma attack.
"It was so frightening as he was too young to tell me what was wrong and how bad he felt.
"Miraj was rushed into Accident and Emergency at our local hospital and was in hospital for three days while they tried to bring his asthma under control using a combination of steroids and antibiotics.
"Miraj now has a flu jab every autumn in preparation for the winter and he has definitely benefited from this.
"I would recommend that anyone who has a child with asthma discusses having the flu jab with their GP. It doesn't make sense to leave it to chance."
Flu jabs are also free to residents in long-stay residential care homes and carers of older or disabled people whose health is at risk if their carer falls ill.
The jab should not be given to people who have a serious allergy to hens' eggs, or who have ever had a serious allergic reaction the flu vaccine in the past.