The flu virus causes influenza (most commonly just called "the flu"). The virus is highly contagious and spreads easily from person to person, mainly when an infected person coughs or sneezes. Because the virus is one of the most changeable of viruses, a new flu vaccine must be produced each year to combat that year's prevalent strains.
Each winter, millions of people suffer from the flu, which is a highly contagious infection. The flu virus spreads easily from person to person, mainly when an infected person coughs or sneezes. The flu virus causes the flu, which is the short name for influenza.
(Click Flu for more information about the flu.)
The flu virus is round, but it can also be elongated or irregularly shaped. Inside the flu virus are eight segments of single-strand RNA (ribonucleic acid) containing the genetic instructions for making new copies of the flu virus. The most striking feature of the flu virus is a layer of spikes projecting from its surface. There are two different types of spikes: one is the protein hemagglutinin (HA), which allows the virus to "stick" to a cell and initiate infection; the other is a protein called neuraminidase (NA), which enables newly formed viruses to exit the host cell.
Influenza viruses are classified as type A, B, or C based upon their protein composition. Type A viruses are found in many kinds of animals, including:
Type B widely circulates in humans. Type C has been found in humans, pigs, and dogs, and causes mild respiratory infections, but does not spark epidemics.
Type A influenza is the most frightening of the three and is believed to be responsible for the global outbreaks of 1918, 1957, and 1968. Type A flu viruses are subdivided into groups based on two surface proteins, HA and NA. Scientists have characterized 16 HA subtypes and 9 NA subtypes of the flu virus.
(Click 1918 Flu for more information about the global outbreak of 1918.)