Influenza is a respiratory infection caused by a family of flu viruses. Often confused with either the common cold or stomach and intestinal infections, most forms of influenza are characterized by a sore throat, headache, chills, body aches, exhaustion, fever, and coughing. Droplets carried from one person to another by sneezing and coughing spread the disease, and touching can also contract it. According to the National Institutes of Health, the disease runs its course in about a week, and can be especially dangerous to infants and toddlers, the elderly, and those with compromised immune systems. Children acquire influenza easily, and can spread it to others rapidly. Normally, peaks of high wintertime reporting and low numbers of cases in the summer characterize influenza trends. Chances of acquiring an influenza infection during the flu season, usually November through March, increase with age, and pneumonia-influenza rates for those over seventy-five years old are much higher than for the middle-aged.

Influenza has annually ranked as one of the ten leading causes of death in the United States. During a typical flu season, anywhere from 35 to 50 million Americans could show symptoms of influenza. About 100,000 of those cases might be hospitalized, and as many as 20,000 could die from influenza and its complications. Influenza is also a substantial threat to human lives in many parts of the third world due to both a lack of effective vaccine distribution programs and a wide range of factors lowering resistance to many viruses.

Because a virus causes influenza, only specific symptoms and some ensuing infections can be treated with antibiotics. Primary prevention in the form of inoculation is the best method of combating influenza. Immunity to particular strains is acquired either naturally by previous exposure or induced with a vaccine, and the variants of influenza change over time. While there are broad categories of influenza A, B, and C that cause illness in humans, the A strains are by far the most lethal and hence the objective for focused vaccine development.

Some vaccines previously used are no longer effective because as the level of immunity in a population increases strains of influenza change genetically and propagate in those susceptible to the altered viral variants. The scholar W. I. B. Beveridge offers the explanation of how an influenza virus capsule has eight separate strands of ribonucleic acid (RNA) for its hereditary material. When a cell of a higher organism is infected by two different varieties of the virus, their sixteen RNA segments can combine to produce progeny with traits from both of the parent strains. New strains capable of evading the host's immunological defenses can thus emerge. Two of the eight viral genes code for protein-sugar complexes on the outer coat of the virus. These surface molecules, hemagglutinin (H) and neuraminidase (N), are antigens that can stimulate the host's immune system to make antibodies against the infecting virus. Subtypes H1, H2, H3, N1, and N2 are known to have caused human influenza epidemics and pandemics (worldwide epidemics) because of major shifts in the order of H and N antigens.

People have suffered from influenza for thousands of years. Written accounts date to the twelfth century, and the development of the printing press combined with changing worldwide travel patterns since the Age of Discovery created more knowledge about the disease. In his Diffision of Influenza (1986), the influenza researcher Gerald Pyle documents pandemics that took place in 1580, 1732–1733, 1800–1803, 1847–1848, and 1889–1892. These and similar events that transpired during the twentieth century are greatly overshadowed by the catastrophic pandemic of 1918–1919. It was so lethal that historians estimate that 40 million people were killed worldwide; a half million of them had been clearly documented in the United States. While the origins of this pandemic are unclear, it emerged from the trenches of World War I in an apocalyptic fashion, leaving a wake of deaths in diffusion pathways extending into most inhabited continents in the world.

Subsequent pandemics have been measured against this event, and although hundreds of thousands in the United States died during pandemics in 1957 and 1968, death tolls were far less than during the 1918–1919 episode. Since the 1950s, vaccine developments have kept pace with genetic shifts of influenza viruses. "Close calls" include a swine flu scare that was averted in 1976 and a contained outbreak of chicken flu in Hong Kong in 1997. According to Gina Kolata, researchers in the late 1990s uncovered the genetic makeup of the 1918–1919 virus from tissue samples stored in paraffin, but that mystery has intensified because the strain identified is unlike any other known influenza viruses. While the biological and geographic origins of the 1918–1919 pandemic remain unclear, it continues to serve as a landmark event in the annals of influenza.

See also: Causes OF Death


Beveridge, W. I. B. "Unravelling the Ecology of Influenza A Virus." History, Philosophy, and Life Science 15 (1993):23–32.

Kolata, Gina. Flu: The Story of the Great Influenza Pandemic of 1918 and the Search for the Virus That Caused It. New York: Farrar, Straus and Giroux, 1999.

Patterson, K. David, and Gerald F. Pyle. "The Geography and Mortality of the 1918 Influenza Pandemic." Bulletin of the History of Medicine 65 (1991):4–21.

Pyle, Gerald. The Diffusion of Influenza: Patterns and Paradigms. Totowa, NJ: Rowman and Littlefield, 1986.

Internet Resources

National Institutes of Health, National Institute of Allergy and Infectious Diseases. "Fact Sheet: Flu." In the National Institute of Allergy and Infectious Diseases [web site]. Available from .


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