Flu starts when an influenza virus enters your body through your nose and mouth, although the virus does not start an infection in everybody who is exposed to it. To make you ill, a virus needs to get into your respiratory tract (the passages from your nose and mouth into your lungs).
The virus can be spread by coughs and sneezes spraying droplets on to surfaces, especially in crowded, enclosed spaces. These viruses can survive outside the body for some time, especially in cold and dry weather. If you touch droplets containing a virus for a respiratory disease, and then touch your face near your nose or mouth, the virus can enter and move down your respiratory tract. This is the most common way to catch the flu.
It usually takes between one and four days for you to develop flu symptoms after the influenza virus enters your body through your nose or throat. You are infectious from the day before you develops symptoms until seven days after symptoms start.