Influenza, or flu, is a respiratory illness caused by a virus.
Flu is highly contagious and is usually spread by the coughs and sneezes of an infected person.
You can also catch flu by touching an infected person (e.g. shaking hands). Adults are contagious 1-2 days before getting symptoms and up to 7 days after becoming ill.
This means that you can spread the influenza virus before you even know you are infected.
Dose: 1 dose every year