Flu (also known as influenza) is a disease of the lungs and upper airways caused by infection with a flu virus. The virus spreads in the lungs and airways. There are three flu viruses, known as A, B and C.

The main symptoms are a high temperature that comes on quickly, and general aches and pains. You may also experience a loss of appetite, nausea and a harsh dry cough. Your symptoms will usually peak after two to three days and you should begin to feel much better within five to eight days, although a cough and general tiredness may last for two to three weeks.

The flu virus is usually spread in the small droplets of saliva coughed or sneezed into the atmosphere by an infected person. Direct contact with hands that are contaminated with the virus can also spread infection.

It takes between one and four days (average two days) to go from being infected to having the full symptoms. People with flu are usually infectious a day before symptoms start and remain infectious for approximately five days after the start of the flu symptoms. Children and people with lowered immune systems may remain infectious for longer. You should therefore try to avoid all unnecessary contact with others during the infectious period.

Flu usually occurs during the winter months (from October to April in the UK). Complications such as a chest infection can affect elderly people or people with certain medical conditions. This can result in serious illness and can be life-threatening.

The number of people who consult their GP with flu-like illness during the winter varies considerably from year to year (usually between 50 and 200 for every 100,000 people). However, healthy people with flu do not need to consult their GP.

An epidemic can be declared if more than 400 people per 100,000 of the population consult their GP with flu or a flu-like illness every week. Flu-like illness describes a variety of other illnesses, which result in similar symptoms.

Source: NHS
http://www.nhs.uk/conditions/flu/Pages/Introduction.aspx