Influenza, commonly called the flu, is a contagious illness caused by one of three types (influenza A, B, and C) of the influenza virus. It is extremely common in the US, and every year 5-20% of the population contracts the flu. Symptoms are generally mild and include fever, headache, coughing, runny nose and muscle aches, however, each year about 36,000 people in the US die from the flu or complications related to the flu. Flu season usually lasts from November until about April or May each year. It is recommended that anyone wishing to avoid contracting the flu be vaccinated, especially those under the age of 5 and over 50, as they are at risk of developing complications from the flu.

There are several different subtypes of the influenza A virus. Each is distinguished from the others by variations in the hemagglutinin (HA) and neuraminidase (NA) proteins displayed on the viral surface. Each combination of HA and NA represents a unique subtype, and the only subtypes known to widely circulate between humans are H1N1, H1N2 and H3N2. As a result of selective pressure and the high mutation rate of the virus, there are a number of variants of each influenza subtype. Some variants produce mild disease and transmissibility, while others produce serious disease and are easily passed from person to person. Occasionally a variant of influenza virus can develop that people have little immunity to and also has extremely facile transmissibility. This deadly combination can lead to a global disease outbreak, called a pandemic. The largest pandemic on record occurred in 1918 with the outbreak of the Spanish Flu.

Influenza is constantly infecting and interacting with both human and animal hosts. Avian influenza subtype H5N1 is currently causing worldwide concern as it has been able to occasionally infect not only birds but also humans. It is referred to in the media as the bird flu and if it develops the ability to spread easily from person to person it could have devastating effects.