Flu is the common name for influenza. It's a virus that infects the respiratory system. Often when you're sick with a virus, your body builds up a defense system by making antibodies against it. That means you usually don't get that particular virus strain again. Unfortunately, flu viruses mutate (change) each year. So you aren't protected from getting the flu forever. Some years the change in the flu virus is slight. So if you do get the flu, it's mild. The antibodies from having the flu before give you partial protection. But every 10 years or so the flu virus goes through a major change and many people get severe cases.

is a viral infection that affects mainly the nose, throat, bronchi and, occasionally, lungs. Infection usually lasts for about a week, and is characterized by sudden onset of high fever, aching muscles, headache and severe malaise, non-productive cough, sore throat and rhinitis. The virus is transmitted easily from person to person via droplets and small particles produced when infected people cough or sneeze. Influenza tends to spread rapidly in seasonal epidemics. Most infected people recover within one to two weeks without requiring medical treatment. However, in the very young, the elderly, and those with other serious medical conditions, infection can lead to severe complications of the underlying condition, pneumonia and death.

  Influenz