A contagious respiratory infec¬tion that frequently occurs in epidemics. The term “flu” is also applied to various forms of viral gastroenteritis, com¬monly known as stomach or intestinal flu, which are unrelated to true respiratory influenza. This article will deal only with the respiratory disease.
History. The so-called “English Sweat” of the 16th century was the first definitely identifiable influenza epidemic. Seemingly, pandemics, or worldwide epidemics, of influenza occur in 20-year to 50-year cycles with minor local epidemics in between. Major pandemics occurred in 1627, 1729, 1788, 1830, 1847, 1872, 1890, 1918, 1957, and 1968. The 1918 pandemic, after World War I, was the worst on record. It killed 10,000,000 more people than the war; in the United States, 500,000 people died. Since World War II vaccines have helped contain flu epidemics and thereby reduce mortality. Mortality was very low in the 1957 pan¬demic, known as the Asian flu because it came from Hong Kong, and was relatively low in the 1968 pandemic. In 1976 there was a flu outbreak in Fort Dix, N.J., that in some cases was caused by the swine flu virus, believed by many experts to have caused the 1918 pandemic. Fearing another disaster, the United States and Canada set up a crash mass vaccina¬tion program. But when some people who had been vacci¬nated developed Guillain-Barre syndrome, a rare type of temporary paralysis, the U.S. program was canceled. A pandemic never developed.
Cause and Epidemiology. Prior to 1933, a bacterium described by Richard F. J. Pfeiffer and named Hemophilus influenzae was suspected to be the cause, but this was never proved conclusively. In the same year, W. Smith, F. W. An-drewes, and P. P. Laidlaw proved the cause of influenza to be a filterable virus which was experimentally transmissible to ferrets. This virus is now identified as influenza A virus. Influenza B virus was discovered in 1940, and influenza C virus was discovered in 1949. At present, the virus is known to exist as these three distinct types. Type A and B viruses mutate rapidly, and each of these types now exists in several strains, or subtypes. The 1957 and 1968 Asian influenza pandemics were caused by strains of type A.
Influenza is spread by direct contact and, for short distances, through air in droplets and dust. Symptoms appear one or two days after exposure. Persons of any age or race or of either sex are equally susceptible to infection by influenza. However, certain groups of persons are more likely to develop complications of influenza, and deaths occur primarily in those over the age of 65 or in those with chronic diseases.
Clinical Course and Diagnosis. The onset of influenza is sudden, with fever, headache, and muscular pains. The throat looks dry and red, and it feels sore. Symptoms such as cough appear on the second or third day. On the third to fifth day, fever drops, characteristically with drenching sweats. Susceptibility to secondary invasion by bacteria is great at this stage. Lassitude and depression may prolong convales¬cence for weeks. An attack confers a specific type immunity.
There is no easy way to diagnose influenza with certainty. The virus can be isolated from the throat and antibodies demonstrated in the blood during convalescence, but these tests are expensive and slow. Diagnosis is usually made on the basis of the occurrence of other similar cases.
Treatment and Prevention. There is no specific cure for influenza, and no known antibiotic has any deterrent effects on any type or strain of the virus. However, antibiotics may be prescribed if secondary bacterial pneumonia occurs. Treatment generally consists of administering medicines to reduce fever and to relieve other symptoms.
The first practical vaccine against influenza was prepared in 1943. It was made with killed viruses and was poly¬valent; that is, it contained viruses of both types A and B. Vaccination was then extended throughout the U.S. armed services, and no serious outbreak of influenza occurred among them during World War II. As new strains of the virus have appeared, they have been incorporated into the vaccine. The U.S. Public Health Service recommends an¬nual vaccination only for those most likely to develop complications from influenza. These include individuals over the age of 65 and anyone who has a chronic debilitating disease, such as heart disease; chronic respiratory disease, such as asthma, bronchitis, or emphysemia; or a chronic metabolic disorder, such as diabetes mellitus.