Influenza

Influenza (or as it is commonly known, the flu or the grippe) is a contagious disease of the upper airways and the lungs, 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. Three influenza pandemics in the 20th century, each following a major genetic change in the virus, killed millions of people.

The term influenza has its origins in 15th century Italy, where the cause of the disease was ascribed to unfavorable astrological influences. Evolution in medical thought led to its modification to "influenza di freddo" (meaning "influence of the cold"), which by the 18th century became the prevalent terminology in the English-speaking world as well.

Contents
1 History
2 Symptoms
3 Flu season
4 Prevention
5 Treatment

History
See also: influenza pandemic
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).

Known epidemics and pandemics - overview
1889-90 - Asiatic Flu, mortality rate said to be 0.75-1 death per 1000 (pandemic)
1918–20 – Spanish Flu, 500 million ill, at least 40 million died (pandemic)
1957–58 – Asian Flu, 1 to 1.5 million died (epidemic)
1968–69 – Hong Kong Flu, 3/4 to 1 million died (epidemic)
Although there were scares in New Jersey in 1976 (the Swine Flu), worldwide in 1977 (the Russian Flu), and in Hong Kong (as well as in other Asian countries, namely continental China, as became known later) 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.

Symptoms
The virus attacks the respiratory tract, is transmitted from person to person by saliva droplets expelled by coughing, and causes the following symptoms:

Fever
Headache
Fatigue/Sore joints (can be extreme)
Dry cough
Sore throat
Nasal congestion
Sneezing
Irritated eyes
Body aches
Extreme coldness
Influenza's effects are much more severe than those of the "cold", and last longer. 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. According to estimates by the World Health Organization, between 250,000 and 500,000 die from influenza infection each year worldwide. 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 a worsening of this condition, that is triggered by the flu.

Flu season
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. Therefore, the World Health Organization (assisted by the National Influenza Centers) 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 promotes transmission from person to person. Another is that cold temperatures lead to drier air, which may dehydrate mucus, preventing the body from effectively expelling virus particles. The virus may also linger longer on exposed surfaces (doorknobs, countertops, etc.) in colder temperatures. Increased travel and visitation due to the holiday season may also play a role. [1]

Prevention
It is possible and in many cases recommended to get vaccinated against influenza with a flu vaccine. 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 which probably will 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. Flu vaccine is available as nasal spray vaccine (recommended for all healthy people ages 5 to 49) and as injectable vaccine.

Treatment
Antiviral treatments that have proven effective in influenza are amantadine, rimantadine, zanamivir, oseltamivir and ribavirin. As most of these substances are expensive, various healthcare organisations and insurers only support their use where this would make a significant difference, e.g. in the elderly.

Worryingly, investigators at the CDC in Atlanta found high rates of resistance to adamantane derivatives (amantadine, rimantadine) in the H3N2 strain of influenza A: China 74%, Hong Kong (70%), Taiwan (23%), South Korea (15%) (Bright et al., 2005). The overall resistance rate in North America was 4%. The enormous rate of resistance in China is believed to be due to the ready availability of amantadine in over-the-counter cold remedies.

A trademarked elderberry extract may aid in shortening the duration of an episode of influenza once contracted, though it has no notable preventive effects (Zakay-Rones et al., 1995).

Children and teenagers with symptoms of influenza should avoid taking aspirin as taking aspirin in the presence of influenza infection (especially influenza type B) can lead to Reye syndrome, a rare but potentially fatal disease of the liver.