Then, when the real thing turns up and tries to enter cells in a vaccinated person, the immune system attacks and destroys before numbers of viruses become large enough to cause an illness.
The race to create this year's flu vaccine starts many months before an outbreak in this country.
The flu virus this year will not be the same as last year's model. As the immune systems of humans become resistant to one strain of the virus, it subtlely changes its clothes, mutating to form another strain.
This means that humans will once again be vulnerable to infection, and have to develop immune resistance from scratch.
One way of doing this is by having a yearly flu jab.
Chicken foetus
This is made up of a sample of a variety of different strains of flu which scientists believe are going to sweep the world that year.
The different viruses are "bred", encouraged to replicate in chick foetus cells, then harvested and inactivated so they can't replicate once injected into humans.
Even in their inactive form, however, what they can do is prime the immune system to recognise them as foreign, and build up its defences.
Then, when the real thing turns up and tries to enter cells in a vaccinated person, the immune system attacks and destroys before numbers of viruses become large enough to cause an illness.
The flu "season" occurs at different times at different locations in the world, as the new mutated virus slowly circles the world.
This means that scientists can look at what is causing illness in Sydney or Hong Kong, and produce a vaccine for the UK.
The flu season in the UK starts in late November to early December, generally peaking some time in January.
The best time to get vaccinated, therefore, is before the middle of November, as it takes two weeks for the vaccine to get the immune system properly primed.
It is a simple injection given into the upper arm, and although it does not give you flu, it can cause some soreness near the injection site for a couple of days, or, less frequently, some minor muscle aching and a slight temperature.
Because it is cultivated using chick foetus cells, anyone with an egg allergy should tell their doctor before having the jab.
Free vaccines
The NHS has decided to drop the age threshold over which people qualify for a free flu jab on the NHS. Now all those aged 65 and over do not have to pay.
Other people in so-called "high risk" groups can also get a free jab. These are:
Other people in line for free jabs include those who work in hospitals or nursing homes, to reduce the chance of their passing the virus on to more vulnerable patients.
However, previous flu vaccination campaigns in the UK have not been particularly successful. Last year, only 40% of those in target groups went to get the vaccine.
The government is distributing 11m jabs this year, and has mounted a far more energetic advertising campaign, featuring former boxer Henry Cooper.
There are signs that this has encouraged more people to have the vaccination.
Those outside the high risk groups can still get inoculated, but will have to pay for the privilege.
Boots the Chemist ran a service last year charging £20 a shot.