The first set of injections aimed at fighting the spread of swine flu are being given out on Wednesday.
Seriously ill people in hospital and the doctors and nurses caring for them will be the first of 14 million people to be given the jab.
For most people the bug is just like a bad cold, but some groups of people are more at risk than others.
The next set of people to be offered the jab include people with health problems and pregnant women.
The government hasn't decided whether everyone in the UK needs to get the injection, but enough medicine has been ordered just in case.
About 500,000 people have been infected with the bug so far.
The number of people with swine flu reached its high point in the summer, but in recent weeks, the number of cases has been growing again.