INFLUENZA

also called Grippe, or Flu, an acute viral infection of the upper or lower respiratory tracts that is marked by fever, chills, and a generalized feeling of weakness and pain in muscles, together with varying signs of soreness in the respiratory tract, head, and abdomen. Influenza is caused by any of several strains of myxoviruses, categorized as types A, B, and C. The type A virus is the most frequent cause of influenza; this type occurs in numerous strains that are clinically differentiated on the basis of the viruses' surface proteins, their geographic origin, date of isolation, and other factors. The major types of influenza viruses generally produce similar symptoms but are completely unrelated antigenically, so that infection with one type confers no immunity against another. Influenza tends to occur in wavelike epidemics throughout the world: influenza A tends to reappear in cycles of two to three years and influenza B in cycles of four to five years; the subtype of influenza A known as H2N2, or Asian flu, for instance, apparently began in East Asia early in 1957, and by midyear it had circled the globe. Influenza may affect individuals of all ages, though the highest incidence of the disease is among children and young adults. Influenza is generally more frequent during the colder months of the year. The influenza infection is transmitted from person to person through the respiratory tract, by such means as inhalation of infected droplets resulting from coughing and sneezing. As the virus particles gain entrance to the body, they selectively attack and destroy the ciliated epithelial cells that line the upper respiratory tract, bronchial tubes, and trachea. The incubation period of the disease is one to two days, after which the onset of symptoms is abrupt, with sudden and distinct chills, fatigue, and muscular aches. The temperature rises rapidly to 38 to 40 C (101 to 104 F). A diffuse headache and severe muscular aches throughout the body are experienced, often accompanied by irritation or a sense of rawness in the throat. In three to four days the temperature begins to fall and the patient begins to recover. Symptoms associated with respiratory tract infection, such as coughing and nasal discharge, become more prominent and may be accompanied by lingering feelings of weakness and debility. Mortality from the disease is commonly low, although it varies in different outbreaks. Death is caused, in most cases, by complications such as pneumonia or bronchitis, usually among older people who are weakened by other debilitating disorders. The antiviral drug amantadine has a beneficial effect on cases of influenza involving the A virus type. Other than this, bed rest and the use of aspirin or other analgesics to control fever remain the standard treatment. Individual protection against influenza may be bolstered by injection of a vaccine of influenza viruses. These viruses are produced in chick embryos and rendered noninfective; standard commercial preparations ordinarily include the type B influenza virus and several of the A subtypes. Protection from one vaccination seldom lasts more than a year, and yearly vaccination may be recommended, particularly for those individuals who are unusually susceptible to influenza or whose weak condition could lead to serious complications in case of infection. Routine immunization in healthy people, however, is no longer generally recommended by the medical profession. Influenza epidemics have a long history; the first clear description of one is that of the epidemic of 1610. The most severe outbreaks in modern times have been those of 188990 and 191819. The Influenza Epidemic of 191819 was the most destructive influenza outbreak in history and one of the severest holocausts of disease ever encountered. It is estimated that 30,000,000 persons throughout the world died of the disease. Subsequent outbreaks of influenza have been much less severe, largely owing to the use of preventive vaccinations.