Flu, or influenza, is a viral infection that mainly affects our respiratory system (nose, throat and lungs). It's extremely variable and changes over time, and different strains are in circulation so a new flu vaccine has to be prepared each year.
Flu usually starts with a sudden fever, muscle aches and pains affecting limbs and joints, feeling very tired and weak, shivering and having the chills, sore throat, a runny or blocked nose, and having a dry and chesty cough.
It can cause severe illness and even death among vulnerable groups including older people or people with an underlying health conditions, and can cause complications for pregnant women.
You can read more about the symptoms of flu on the NHS website.
How does the flu jab work?
The flu jab stimulates your body's immune system to make antibodies to attack the flu virus. Antibodies are proteins that help fight off germs, such as viruses, that have invaded your blood.
If you're exposed to the flu virus after you've had the flu jab, your immune system will recognise the virus and produce antibodies to fight it.
It may take 10 to 14 days for your immunity to build up fully after you've had the flu vaccine.
In February each year the World Health Organisation (WHO) assesses the strains of flu virus that are most likely to be going round over the following winter months. Based on this assessment WHO recommends which flu strains the flu jab should contain each winter.
Who should get the flu jab?
The NHS have a list of eligible groups of people who should have their flu jab every year and for these groups the flu jab is free from your GP or local pharmacy.
If you're not eligible for a flu jab through the NHS, it's still a good idea to get the jab as it’s the best way to protect yourself and others. You can pay to have a flu jab at most high street pharmacies.