Influenza (or as it is commonly known, the flu or the grippe) is a contagious disease, caused by an RNA virus of the orthomyxoviridae family. It rapidly spreads around the world in seasonal epidemics, imposing considerable economic burden, in the form of health care costs and lost productivity. Major genetic changes in the virus caused three influenza pandemics in the 20th century, killing millions of people, and avian influenza has been identified as the most likely source of a future pandemic.
The name comes from the old (incorrect) medical belief that unfavourable astrological influences cause the disease.
There are three genuses of the virus, identified by antigenic differences in their nucleoprotein and matrix protein:
- Influenza A viruses that infect mammals and birds (also known as avian influenza)
- Influenza B viruses that infect only humans
- Influenza C viruses that infect only humans
The A type of influenza virus is the type most likely to cause epidemics and pandemics. This is because the influenza A virus can undergo antigenic shift and present a new, immune target to susceptible people. Populations tend to have more resistance to influenza B and C, because they only undergo antigenic drift, and have more similarity with previous strains.
Influenza A viruses can be further classified, based on the viral capsid proteins hemagglutinin (HA or H) and neuraminidase (NA or N) that are essential to the virus' life cycle. Sixteen H subtypes and nine N subtypes have been identified for influenza A virus. Only one H subtype and one N subtype have been identified for influenza B virus. At present, the most common antigenic variants of influenza A virus are H1N1 and H3N2. (Yohannes et al., 2004)
Yet further variation exists; thus, specific influenza strain isolates are identified by a standard nomenclature specifying virus type, geographical location where first isolated, year of isolation, sequential number of isolation, and HA and NA subtype (Yohannes et al 2004) Examples of the nomenclature are A/Moscow/10/99 (H3N2) and B/Hong Kong/330/2001.
The term superflu is used to refer to a strain of flu that spreads unusually quickly, is unusually virulent, or for which the host is uncommonly unresponsive to treatment— the kinds of strains which cause epidemics or pandemics. There is no exact scientific definition of a superflu.
There were several serious outbreaks of influenza in the 20th century. The most famous (and the most lethal) was the so-called Spanish Flu pandemic (type A influenza, H1N1 strain), which lasted from 1918 to 1919, and is believed to have killed more people in total than World War I. While the war casualties accumulated over several years, the pandemic took most of its toll over a period of weeks. Lesser flu epidemics included the 1957 Asian Flu (type A, H2N2 strain) and the 1968 Hong Kong Flu (type A, H3N2 strain).