INFLUENZA VIRUS A

GROUP - V

ORDER - None assigned

FAMILY - Orthomyxoviridae

GENUS - Influenzavirus A

GENOME - Linearsingle-stranded RNA in eight segments totaling about 14,000 nucleotides, and encoding 11 proteins

GEOGRAPHY - Worldwide

HOSTS - Humans, pigs, waterfowl, chickens, horses, dogs

ASSOCIATED DISEASES - Influenza, or the flu

TRANSMISSION - Contact, airborne

VACCINE - Live attenuated and inactivated, to multiple seasonal strains

Source Wikipedia

Ever-changing virus strains prevent lifelong immunity Seasonal flu is a dreaded disease, and some strains have caused severe pandemics, the most famous being the so called Spanish flu, or the 1918 pandemic. About 40,000,000 people died worldwide in the 1918 pandemic. Many deaths were from secondary bacterial infections, in the era before antibiotics. There were probably many severe pandemics before 1918, before we knew that influenza was a virus. Influenza virus is endemic in waterfowl around the world, and does not cause disease in these birds. It is only when it moves into mammalian hosts, especially pigs and humans, that it is a major problem. It can also cause a problem in domestic birds such as chickens, and a few notable strains of the virus have moved into humans directly from these birds. These strains are particularly severe, and sometimes have a high deathrate , but thus far they need not gained the power to transmit from human to human.

Source Wikipedia

Strains are often referred to as HxNx (for example, H1N1, and H3N2), referring to the proteins on the outside of the virus that elicit the major immune response. Because these proteins are encoded on different RNAs the virus can sometimes mix and match segments during a mixed infection, to become a replacement strain that's novel to the human immune system. These mixed infections often occur in pigs, which then transmit the virus to farm workers, and therefore the human infection cycle starts. These new strains are called antigenic shifts, and are usually the cause of pandemics. Between pandemics the virus mutates more gradually, causing antigenic drift. This creates the necessity for brand spanking new flu vaccines annually , supported the present strains in circulation. Because the vaccine must be produced before the flu season begins, evolutionary biologists carefully study the trends of influenza virus evolution to project what the antigens are going to be for the coming season. These projections are not always accurate; this is why the vaccines have varying levels of effectiveness from year to year. Getting the flu can give a broader immunity that lasts for several years.