Flu
(Influenza)

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Flu (influenza, conventional, H1N1, H3N2, and bird flu [H5N1]) facts

  • Influenza, commonly called "the flu," is caused by viruses that infect the respiratory tract.
  • Influenza viruses are divided into three types, designated A, B, and C, with influenza A types usually causing the most problems in humans.
  • Most people who get the conventional or seasonal flu recover completely in one to two weeks, but some people develop serious and potentially life-threatening medical complications, such as pneumonia.
  • Much of the illness and death caused by conventional or seasonal influenza can be prevented by annual influenza vaccination.
  • Influenza A undergoes frequent antigenic changes that require new vaccines to be developed and people to obtain a new vaccination every year. New vaccine technology is being developed.
  • In April 2009, a new flu virus termed novel H1N1 swine flu developed in Mexico, rapidly spread worldwide, and caused the WHO to declare a flu pandemic. Eventually, the WHO declared the pandemic over in 2010. In 2012, a new type of flu strain developed, H3N2v, but has not developed into any epidemic situations currently.
  • Like the influenza virus, drug treatments are constantly changing and improving, but currently, timely vaccination is still considered to be the best defense against the flu. However, the CDC considers antivirals an important adjunct to the flu vaccine in the control of the disease process.
  • People should be aware that flu pandemics can cause severe flu symptoms and sometimes cause death in many individuals who may be more susceptible to the pandemic flu than the conventional flu; however, the previous pandemic flu virus (H1N1) has been available in vaccines and is considered part of the conventional circulating flu viruses.
  • Bird flu (H5N1) mainly infects birds but it also infects humans who have close contacts with birds.
  • Individuals should check with their doctors to determine if they are considered to be at higher risk of getting severe flu symptoms than the normally healthy population.
Picture of the influenza virus
Picture of the influenza virus

What is influenza?

Influenza, commonly called "the flu," is an illness caused by RNA viruses that infect the respiratory tract of many animals, birds, and humans. In most people, the infection results in the person getting a fever, cough, headache, and malaise (tired, no energy); some people also may develop a sore throat, nausea, vomiting, and diarrhea. The majority of individuals has symptoms for about one to two weeks and then recovers with no problems. However, compared with most other viral respiratory infections, such as the common cold, influenza (flu) infection can cause a more severe illness with a mortality rate (death rate) of about 0.1% of people who are infected with the virus.

The above is the usual situation for the yearly occurring "conventional" or "seasonal" flu strains. However, there are situations in which some flu outbreaks are severe. These severe outbreaks occur when a portion of the human population is exposed to a flu strain against which the population has little or no immunity because the virus has become altered in a significant way. These outbreaks are usually termed epidemics. Unusually severe worldwide outbreaks (pandemics) have occurred several times in the last hundred years since influenza virus was identified in 1933. By an examination of preserved tissue, the worst influenza pandemic (also termed the Spanish flu or Spanish influenza) occurred in 1918 when the virus caused between 40-100 million deaths worldwide, with a mortality rate estimated to range from 2%-20%.

In April 2009, a new influenza strain against which the world population has little or no immunity was isolated from humans in Mexico. It quickly spread throughout the world so fast that the WHO declared this new flu strain (first termed novel H1N1 influenza A swine flu, often later shortened to H1N1 or swine flu) as the cause of a pandemic on June 11, 2009. This was the first declared flu pandemic in 41 years. Fortunately, there was a worldwide response that included vaccine production, good hygiene practices (especially hand washing) were emphasized, and the virus (H1N1) caused far less morbidity and mortality than was expected and predicted. The WHO declared the pandemic's end on Aug. 10, 2010, because it no longer fit into the WHO's criteria for a pandemic.

A new influenza strain, H3N2, was identified in 2011, but this strain has caused only about 330 infections with one death in the U.S. Another strain, H5N1, a bird flu virus, has been identified since 2003 and has caused about 650 human infections; this virus has not been detected in the U.S. and currently is not known to be easily spread among people in contrast to other flu strains. Unfortunately, people infected with H5N1 have a high death rate (about 60% of infected people die); currently, H5N1 is not readily transferred from person to person like other flu viruses.

Haemophilus influenzae is a bacterium that was incorrectly considered to cause the flu until the virus was demonstrated to be the correct cause in 1933. This bacterium can cause lung infections in infants and children, and it occasionally causes ear, eye, sinus, joint, and a few other infections, but it does not cause the flu.

Medically Reviewed by a Doctor on 9/23/2014

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What to Do if You Get the Flu

Influenza, or the "flu," is a illness of the breathing system (respiratory system) and muscles caused by a virus. While a vaccine is available to prevent the flu, its effectiveness varies according to the degree of match between the viral strains used to prepare the vaccine and those strains actually in circulation in a given year. Not everyone receives the flu vaccine, and even some of those who do can develop symptoms of the flu.