Influenza, commonly referred to as the flu, is a contagious (able to be spread) infection of the respiratory system that is caused by viruses, including influenza type A, B, and C.
Influenza viruses are transmitted through the air in tiny droplets when someone with the infection coughs, sneezes, or talks. Individuals are then exposed to the virus through inhalation, or by contact with objects such as telephones, door handles, railings, or computer keyboards. An infection may occur when the virus is then transferred to the eyes, nose, or mouth.
In general, the flu is more debilitating than the common cold, and symptoms such as fever, body ache, extreme fatigue (tiredness), and dry cough are seen more in the flu and are more severe. Some influenza viruses can even cause death in otherwise healthy individuals.
Although many individuals confuse common colds with influenza because they both affect the upper respiratory system and present with similar symptoms, they are actually caused by different viruses. The upper respiratory system includes the nose, trachea (windpipe), throat, sinuses (air filled spaces in the skull), bronchial tubes (leads from the trachea to the lungs), and larynx (voice box).
According to the Centers for Disease Control (CDC), every year in the United States (U.S.), on average five to 20% of the population gets the flu and more than 200,000 people are hospitalized from flu complications, such as dehydration (loss of water), high fever (over 102 degrees Fahrenheit), and extreme fatigue. About 36,000 people die from flu every year.
Some patients, such as older people, young children, and people with certain health conditions that lower immunity, including cancer, human immunodeficiency virus (HIV), and acquired immunodeficiency syndrome (AIDS), are at high risk for serious flu complications.
Children are two to three times more likely than adults to get sick with the flu. Also, children frequently spread the virus to others due to bad hygiene, such as sneezing without covering the nose.
Treatment for the flu includes bed rest and plenty of fluids, along with symptomatic treatment such as drugs to fight viral infections, reduce fever, and help with sore throat and cough. Prevention includes an influenza vaccine.
Flu epidemics: When a flu epidemic occurs, specific populations are infected with a type of influenza virus that has not been encountered before. The immune system of most of the general population cannot respond to the viral infection, and widespread illness and even death can occur. Some individuals, for reasons unknown, may be immune. Epidemics may be restricted to one locale (an outbreak), more general (an "epidemic"), or even global (pandemic).
Bird flu: An emerging type of virus infecting humans is the avian influenza virus, or bird flu. Several cases of human infection with bird flu viruses have occurred since 1997, mostly in Asia. No human infections with avian influenza A virus have been identified in the United States. The CDC estimates that the death rate for these reported cases has been about 50% in infected humans and 100% in birds. This virus is mainly transmitted to humans through direct contact with live or dead poultry; however, it is thought that a few cases of human-to-human spread have occurred.
H1N1: H1N1 is a subtype of the type A influenza virus that causes an illness commonly known as the flu. The H1N1 subtype currently holds much interest because an ongoing pandemic involves a novel (new) strain of the virus. This outbreak began in late March of 2009 in Mexico and is predicted to continue into the regular flu season of 2009-10. According to the U.S. Centers for Disease Control (CDC), as of September 2009, around 9,000 people in the United States have been hospitalized due to novel H1N1 infection, and fewer than 600 deaths are linked to this new virus. It is estimated that a total of one million people have contracted H1N1 flu in the United States. In the southern hemisphere, the seasonal flu period is between April and November. By late August of 2009, the levels of influenza (including H1N1) had returned to normal in the southern hemisphere, according to the World Health Organization.
This novel strain was originally referred to as "swine flu" because it appeared similar to influenza viruses that normally infect pigs (swine) and rarely infect humans. There was widespread concern that eating pork or coming into contact with pigs would increase the risks of contracting the novel H1N1 flu. However, later studies showed that this novel H1N1 strain (henceforth, H1N1 or novel H1N1) is quite distinct from influenza viruses that normally infect pigs. It appears that the virus is derived from multiple viruses that may have originated in swine. Novel H1N1 is spread from humans to humans and is not known to be transmitted by eating pork or from close contact with swine. It has not yet been identified in pigs in the United States, and only isolated herds of swine have been infected around the world. Thus, the term "swine flu" is somewhat of a misnomer. Nonetheless, it is still currently used to refer to novel H1N1.
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