Flu viruses spread mainly by tiny respiratory droplets made when people with flu cough, sneeze or talk. These droplets can be ingested in the mouths or noses of people who are nearby.
A person might also get flu by touching a surface or object that has flu virus on it and then touching their own mouth, nose or possibly their eyes. The virus can live for around 24 hours on things such as computer keyboards, handrails, door handles, and so on. Good personal hygiene and cleaning of surfaces can help to prevent the spread of flu.
You may be able to spread flu to someone else before you know you are sick, as well as while you are sick. People with flu will be infectious for a day before symptoms develop and for a total period of about a week.
Around 1 in 3 people infected by the flu virus will not show any symptoms, but they can still pass it on to others.