How it is spread
The flu virus is spread in the small droplets of saliva coughed or sneezed into the air by an infected person. If you breathe in these droplets, you may become infected.
Flu can also spread if someone with the virus touches common surfaces such as door handles with unwashed hands.

The infectious period
Symptoms develop one to four days (two days on average) after being infected.
People with flu are usually infectious (can spread the virus) a day before symptoms start, and remain infectious for five or six days. Children and people with weaker immune systems (such as cancer patients) may remain infectious for slightly longer.
Try to avoid all unnecessary contact with others during this infectious period.

A video of how flu spreads
This video is quite good, it gives an intro as to how the flu enters your immune system and reproduce itself.

How common is it?
Seasonal flu is a very common illness that occurs every year, usually during the winter months (October to April in the UK).
The number of people who consult their GP with flu-like symptoms varies from year to year, but is usually between 50 and 200 for every 100,000 people. This is in addition to the many people with flu who do not see their GP.

Outlook
Your symptoms will usually peak after two to three days. You should begin to feel much better within five to eight days.
However, elderly people or those with certain medical conditions may develop a complication such as a chest infection. This can lead to serious illness and can be life-threatening.
In the UK, about 600 people a year die from seasonal flu. This rises to around 13,000 during an epidemic.
A seasonal flu vaccine is available free if you are over 65, have a serious medical condition or live in a residential home. For more information see Seasonal flu jab.