Expectant mothers and young children are being urged to get the flu vaccine before the winter sets in, as part of a major public health drive.
Public Health England (PHE) warned mums-to-be that pregnancy naturally weakened the immune system and urged them to visit their GP to get the jab.
ITV News Reporter Sally Biddulph reports:
Young children between the ages of two and four are eligible for the new nasal vaccine spray, which should protect them against the disease.
Flu has little effect beyond a week's sickness for most adults and most cases can be treated at home without a visit to the doctor, PHE said.
Children under the age of 17, pregnant women and the over-65s are at risk of developing a more serious disease, like pneumonia, if they put off getting vaccinated or visiting the doctor.
How to recognise the signs of and prevent flu:
Flu is transferred through the sneezes and coughs of an infected person, so those close to a sick person should take care to wash their hands regularly.
Symptoms hit quickly and include fever, chills, headaches and aching muscles.
Young children with an egg allergy may well be allergic to the jab and parents should speak to their family doctor before giving one to their child.
Healthy adults do not need to visit their GP if they start exhibiting flu-like symptoms. The virus can be treated by getting rest, staying warm and keeping hydrated.
The flu jab works best if the patient is immunised some point between the beginning of October and end of November.
However, those in the at-risk groups are still encouraged to have the flu jab even if they have missed this window.
The Centers for Disease Control and Prevention say flu vaccines are designed to protect against the main flu viruses that research suggests will be the most common during the upcoming season.
Three kinds of flu viruses commonly circulate among people today: Influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses.
PHE says the reason a flu jab is important every year is because "flu is an unpredictable virus" it adds that "new strains might circulate each year with varying intensity".
How the flu virus can shift and change:
'Antigenic drift' - small changes in the genes of influenza viruses that happen continually over time as the virus replicates.
'Antigenic shift'- an abrupt major change in the influenza A viruses resulting in a new influenza A subtype, or a virus emerging from an animal population that is so different from the same subtype in humans that most people do not have immunity, eg H1N1 in spring 2009.
. Type A viruses undergo both 'drift' and 'shift' changes but influenza type B viruses change only by the more gradual process.
The seasonal flu campaign, which was launched on 6 October, will run for four weeks.