Influenza is an infectious disease caused by the influenza virus. The term influenza is used to describe influenza and other similar illnesses. Thegeneral symptoms of influenza, also known as the flu or grippe, are chills,headache, fever, weakness and aching of the joints. The incubation period is short--between one and three days--and the first symptom is a fever that may reach 103° F. Cough and gastrointestinal discomfort may also accompany the disease. Usually, the viral infection runs its course in about a week. However, the virus weakens the immune system, making the human body subject to secondary infections such as bacterial pneumonia. Fatalities associatedwith influenza usually result from such secondary complications. The presenceof viral pneumonia was probably the cause of many deaths during the great influenza epidemics of the past.

Historically, influenza has been known for centuries and may have been what Hippocrates of Cos described as the cause of an epidemic as early as 412 b.c. In the sixteenth century, John Keys (1510-1573), also known byhis Latinized name, Johannes Caius, was a well-educated English physician who wrote a treatise on the "Sweatyng Sickness." In this curious book hedescribes a highly contagious illness in which death frequently occurred within a few days or even hours after symptoms began to show. The symptoms he described included fever, delirium and labored breathing--not very different from influenza symptoms. At that time the cause was sometimes attributedto the English diet--but nobody really knew how the sweating sickness was transmitted. Treatment included bed rest and doing anything that would promotesweating.

Even at the beginning of the twentieth century, doctors had difficulty determining how influenza spread. They knew it was not spread by bacteria from unsanitary conditions, nor by insects. But what doctors did know was that the disease could be fatal. Between 1918 and 1919, influenza killed about 20 millionpeople all over the globe. In the United States more than 550,000 people died of the flu--ten times more Americans than were killed in action during World War I. There were emergency hospitals set up to treat flu patients. Soldiers were made to gargle with salt water to prevent the disease.

Scientists and physicians searched for the cause of influenza but could not find one. Not until the 1930s with the invention of more powerful microscopesdid scientists began to see pathogens much smaller than bacteria--viruses. During that decade the nature of viruses was discovered. Wendell Stanley, an American biochemist, prepared large quantities of viruses, and found that theycould be crystallized. Viruses are very simple structures made of only proteins and nucleic acids which could be crystallized in much the same way as other nonliving chemicals. However, when a virus is inside a living cell it usesthe cell's genetic machinery to make more copies of itself. Stanley had discovered that viruses are on the borderline between living and nonliving thingsbecause they grow only when inside living cells. During World War II, Stanleyworked on culturing the influenza virus. Today influenza is grouped with other viruses known as adenoviruses.

Because viruses are so tiny, vast numbers are found in human body fluids likemucus and saliva. Coughing or sneezing releases an aerosol spray filled withviruses. The pathogen easily travels from one person to another, making influenza very contagious. The viruses usually enter the upper respiratory tractand begin reproducing. The body's immune system, including antibodies and T lymphocytes, work to kill the viruses, but may be not be enough to stop the disease from running its course.

Scientists are still puzzled by the influenza outbreak that occurred around the time of the First World War. Originating in the American Midwest, the epidemic spread world wide, possibly carried abroad by American soldiers during World War I. Incredibly, lung tissue from an Army private who died from the flu was preserved in paraffin and stored in Washington, DC. In 1997, scientistsused polymerase chain reaction--which amplifies minute genetic material--toextract the virus. Genetic studies found the flu contained a pig virus. The study further proved that pigs, which are susceptible to both bird and human viruses, can serve as a cauldron where new and dangerous viral strains can intermingle and form other strains. The finding also dispelled a long-time belief that the epidemic may have developed from an avian, or bird, virus.

In 1997 a three-year-old boy in Hong Kong died from a virus previously thought to occur only in birds. Investigators believe that the transmission was direct from bird to human without the usual intermediary pig. Prior to the boy'sinfection, thousands of chickens in Hong Kong died from the virus. By the end of 1997, 15 additional cases had occurred in the city.

Research has shown that viruses have a phenomenal ability to change, or mutate, very quickly. A treatment that may have worked on one flu virus may not work on another. Each year different strains develop and are named for places where they first occur. The Asian flu caused a worldwide epidemic in 1957-1958and the Hong Kong flu in 1968-1969. Fewer deaths resulted because antibiotics were used to treat secondary infections.

The best treatment for influenza is prevention; and flu vaccines (both killedand live virus) are recommended, especially for the very young and the veryold. In addition the antiviral drug known as amantadine has been used to treat certain strains of influenza.

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