Influenza, commonly called the flu, is a highly contagious infection of the respiratory tract that spreads from person-toperson through infectious respiratory secretion droplets caused by coughing or sneezing. Influenza outbreaks occur almost every year and their severity varies considerably. One unique aspect of influenza compared with other viruses, is its ability
to continuously change over time, usually by mutation. This characteristic enables the virus to evade the immune system of its host, making people susceptible to the flu throughout their entire life. When infected with the virus, a person develops an antibody that works against that virus. Once the virus changes, however, the previous antibody is unable to recognize it, necessitating an entirely new antibody to fight off the virus. These modifications make it necessary for individuals to receive a different influenza vaccination each year, compared with one vaccination that would grant lifetime immunity.

Morbidity and Mortality
Each year approximately 10-20% of the world's population contracts influenza and on average 3 to 5 million people suffer severe illness. An estimated 250,000 to 500,000 people die annually from influenza-related complications. Occasionally a major genetic shift in the influenza virus results in a deadly new virus strain to which the human population does not have immunity, and a global pandemic outbreak occurs. The Spanish influenza pandemic, the most severe outbreak of influenza to date, occurred from 1918 to 1920 and caused deaths worldwide ranging in estimation from 20 to 60 million. According to the World Health Organization, as of January 11, 2007, 264 people had been infected with H5N1 virus, resulting in 150 deaths.

While the flu affects individuals of all ages, approximately 90% of flu-related deaths occur among individuals above the age of 65. People with chronic medical conditions and young children also have a higher risk of suffering influenza complications. 

Geographical Distribution
The influenza virus occurs worldwide. In temperate regions, influenza occurs during the winter months, affecting the northern hemisphere from November to March and the southern hemisphere from April to September. In tropical regions influenza may occur at any time of the year.

Transmission
Influenza transmission occurs mostly by breathing in respiratory droplets that enter the air when an infected person coughs or sneezes. Transmission also occurs through direct contact with respiratory droplets or secretions, followed by touching the nose or mouth.