- This article concerns the respiratory diseases properly named influenza. For the digestive-system diseases commonly called "stomach flu" (and widely misunderstood as being instances of influenza), see gastroenteritis.
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 have caused three influenza pandemics in the 20th century, killing many millions of people. The name comes from the old medical belief in unfavourable astrological influences as the cause of the disease.
There are three types of the virus:
- Influenza A viruses that infect mammals and birds
- 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 haemagglutinin (HA or H) and neuraminidase (NA or N) that are essential to the virus' life cycle. Fifteen 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.
There were several serious outbreaks of influenza in the 20th century. The most famous (and the most lethal) was the 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. Lesser flu pandemics included the 1957 Asian Flu (type A, H2N2 strain) and the 1968 Hong Kong Flu (type A, H3N2 strain).
Although there were scares in New Jersey in 1976 (the Swine Flu), worldwide in 1977 (the Russian Flu ), and in Hong Kong in 1997 (Avian influenza), there have been no major pandemics subsequent to the 1968 infection. Increased immunity from antibodies and the development of flu vaccines have limited the spread of the virus and so far prevented any further pandemics.
The virus attacks the respiratory tract, is transmitted from person to person by saliva droplets expelled by coughing, and causes the following symptoms:
- Tiredness (can be extreme)
- Dry cough
- Sore throat
- Irritated eyes
- Body aches
- Extreme coldness
Influenza's effects are much more severe and last longer than those of the cold. Recovery takes about one to two weeks. Influenza can be deadly, especially for the weak, old or chronically ill. Some flu pandemics have killed millions of people.
Most people who get influenza will recover in one to two weeks, but others will develop life-threatening complications (such as pneumonia). Millions of people in the United States (about 10% to 20% of U.S. residents) are infected with influenza each year. An average of about 36,000 people per year in the United States die from influenza, and 114,000 per year are admitted to a hospital as a result of influenza. Even healthy people can be affected, and serious problems from influenza can happen at any age. People age 65 years and older, people of any age with chronic medical conditions, and very young children are more likely to get complications from influenza. Pneumonia, bronchitis, sinus, and ear infections are four examples of such complications.
The flu can make chronic health problems worse. For example, people with asthma may experience asthma attacks while they have the flu, and people with chronic congestive heart failure may have worsening of this condition that is triggered by the flu.
Influenza is an extremely variable disease; similar viruses are found in pigs and domestic fowl. In areas where there are high concentrations of humans, pigs and birds in close proximity, such as parts of Asia, simultaneous infections across species enable genetic material to be exchanged between the various strains of flu. This appears to be the principal method by which new infectious strains arise. It is believed that sooner or later, a recombination may occur to produce a strain as lethal as the 1918 virus. In late 1997, a new strain of avian influenza (also known as bird flu) originating from chickens infected 18 people in Hong Kong, of whom 6 died. This strain did not appear to be readily transmissible from human to human, but such a high mortality rate, and the possibility of a further recombination to make it more infectious, meant that the risk was considered so great that all domestic poultry in Hong Kong was slaughtered. Avian influenza transmissible to humans resurfaced in January 2004 in Cambodia, Vietnam, and Thailand.
Influenza reaches peak prevalence in winter, and because the Northern and Southern Hemisphere have winter at different times of the year there are actually two flu seasons each year. Hope-Simpson (1981) observed that influenza outbreaks are globally ubiquitous and consistently occur six months following the time of maximum solar radiation in an area. Therefore the World Health Organization makes two vaccine formulations every year, one for the Northern and one for the Southern Hemisphere.
While most influenza outbreaks in the Northern Hemisphere tend to peak in January or February, not all do. For example, the influenza pandemic of 1918 and 1919 reached peak virulence during late spring and summer worldwide, and not until October in the US. It remains unclear why outbreaks of the flu occur seasonally rather than uniformly throughout the year. One possible explanation is that, because people are indoors more often during the winter, they are in close contact more often and this is enough to trigger the outbreak. Another is that the cold weakens the immune system; however, the virus is contracted in a warm indoor environment in which it can thrive.
It is possible to get vaccinated against influenza, however, due to the high mutability of the virus a particular flu vaccine formulation usually only works for about a year. The World Health Organization co-ordinates the contents of the vaccine each year to contain the most likely strains of the virus to attack the next year. The flu vaccine is usually recommended for anyone in a high-risk group who would be likely to suffer complications from influenza.
An Israeli study has also shown that Sambucol , a trademarked elderberry extract, may aid in curing influenza once contracted, though it has no notable preventative effects.
The term "stomach flu"
Many people use the term "stomach flu" to describe illnesses with nausea, vomiting, or diarrhea. These symptoms can be caused by many different viruses, bacteria, or even parasites. While vomiting, diarrhea, and nausea can sometimes be related to influenza, particularly in children, these problems are rarely the main symptoms of the infection. Influenza is a respiratory disease and not a stomach or intestinal disease. See gastroenteritis for one cause of these symptoms.
- Hope-Simpson RE (1981). The role of season in the epidemiology of influenza. J Hyg (Lond) 86 (1), 35-47. PMID 7462597
- Yohannes K, Roche P, Hampson A, Miller M, Spencer J (2004). Annual report of the National Influenza Surveillance Scheme, 2003. Commun Dis Intell 28 (2), 160-8. PMID 15460951
Some of this article was originally from the public domain CDC publication http://www.cdc.gov/ncidod/diseases/flu/fluinfo.htm