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influenza or flu, acute, highly contagious disease caused by a RNA virus (family Orthomyxoviridae); formerly known as the grippe. There are three types of the virus, designated A, B, and C, but only types A and B cause more serious contagious infections. Influenza is difficult to diagnose in the absence of an epidemic, since it resembles many common respiratory ailments. It can be distinguished from a cold, however, by sudden fever, prostration, weakness, and sometimes severe muscular aches and pains. Stomach and intestinal symptoms, such as nausea and vomiting, are not commonly due to influenza infection, and the term stomach flu is a misnomer. Influenza is usually self-limiting, but complications such as pneumonia and bronchitis can be serious threats to newborns, the elderly, and people with chronic diseases. The viruses are spread by respiratory droplets, and the disease is typically most widespread from the late fall to early spring.

Vaccination is recommended for persons who are likely to be exposed to influenza (such as health-care workers) or who are at risk for complicatons. The antiviral drugs amantadine and rimantadine are effective against most strains of type A influenza, and zanamivir and oseltamivir against types A and B. Given within two days of the first appearance of symptoms, they may reduce the symptoms; they may also be given to prevent influenza infection in persons exposed to the disease. Uncomplicated influenza requires only rest and treatment of symptoms, and the use of antibiotics has greatly reduced fatalities from secondary infections. Return to normal activity should be undertaken slowly, as relapses are easily precipitated.

Serious influenza in humans is caused by strains of several A subtypes (which are designated by the specific combination of the 19 hemagglutinin and 9 neuraminidase proteins, or antigens, found on the virus's surface, e.g., H1N1) and by strains of type B. Type A is also found in swine, horses, whales, seals, and other animals, but wild birds are the only animals to have all A subtypes, and migratory birds can spread a strain of the disease great distances. Some H5 and H7 strains of avian influenza (also called avian flu or bird flu) are especially virulent and can result in financially devastating losses in the poultry industry. As a result, outbreaks of the disease are usually controlled by severe measures, including killing all poultry within a couple miles of the outbreak. Avian and swine influenza occasionally infect humans, but such cases rarely result in human-to-human transmission.

The influenza vaccine, which is based on the hemagglutinin and neuraminidase proteins, confers immunity only to the particular strain or set of strains including in the vaccine, and immunity to one strain or subtype, whether acquired through infection or vaccination, does not prevent susceptibility to another. Because the surface antigens of flu viruses change over time, it is necessary to reformulate the vaccine yearly in an educated guess at what strain will appear. (An influenza A vaccine that utilizes a surface protein that does not mutate is under development.) Abrupt major changes in a virus, which can result in increased virulence, also occur. Swine, which can be infected by avian and human influenzas, can facilitate such a development when avian and human strains are both present in an animal, enabling the genetic material of the two to reassort (mix). A major change can similarly occur in a person who is infected by both human and avian viruses.

Epidemics of influenza may be caused by type A or B strains, although type B is more likely to occur sporadically. Pandemics (worldwide epidemics) are caused only by type A. Three such pandemics occurred in the 20th cent., in 1918–19 (the “Spanish flu”), 1957–58 (the “Asian flu”), and 1968–69 (the “Hong Kong flu”). In 1918–19, some 675,000 people died in the United States, and between 50 and 100 million died worldwide. Research suggests that the 1918–19 strain arose when an avian strain acquired the ability to infect humans, and the other two pandemics are known to have been caused by strains produced by the reassorting of human and avian viruses.

The avian strain A (H5N1), first known to have been transmitted directly to humans in 1997, began a new outbreak in several E Asian nations in 2003 and has shown increased virulence when transmitted to humans. International health officials are concerned that it could reassort with a human influenza virus, resulting in a new strain that would be both extremely virulent and highly contagious. By early 2006 the A (H5N1) outbreak had spread across Asia to birds and poultry in many European and some Africa nations. More than 700 cases of A (H5N1) influenza have been identified in humans, largely in Asia; roughly 60% of the cases have been fatal. Another avian strain, A (H7N9), infected humans with often deadly results in 2013–14. This strain, which first appeared in China in 2013 and has been associated with a series of outbreaks since then, has been transmitted to humans mainly from poultry; there have been rare cases of human-to-human transmission. More than 1,600 cases have been reported since 2013, with most resulting in pneumonia; some 40% of patients have died.

A new A (H1N1) strain of human influenza, containing genetic material from both swine, avian, and human influenzas but popularly known as swine flu, was detected in patients in Mexico in Apr., 2009, and rapidly spread worldwide, officially becoming pandemic by June, when at least 29,000 people in 74 nations had been infected, though many more uncounted cases were believed to have occurred. Apparently no more severe in most people than the normal seasonal flu, it nonetheless demonstrated how quickly a new strain to which many humans had little resistance could be spread around the globe, and by Nov., 2009, had become the dominant strain of human influenza worldwide. The outbreak was downgraded from pandemic status in Aug., 2010.


See G. Kolata, Flu (1999) and J. Brown, Influenza (2018); studies of 1918–19 pandemic by A. W. Crosby (2d ed. 2003), J. M. Barry (2004), L. Spinney (2017), and C. Arnold.

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An acute respiratory viral infection characterized by fever, chills, sore throat, headache, body aches, and severe cough. The term flu, which is frequently used incorrectly for various respiratory and even intestinal illnesses (such as stomach flu), should be used only for illness with these classic symptoms. The onset is typically abrupt, in contrast to common colds which begin slowly and progress over a period of days. Influenza is usually epidemic in occurrence. The first documented pandemic, or global epidemic, of influenza is considered to have been in 1580. The influenza pandemic of 1918, the most famous occurrence, was responsible for at least 20 million deaths worldwide.

The three types of influenza viruses, types A, B, and C, are classified in the virus family Orthomyxoviridae, and they are similar, but not identical, in structure and morphology. Types A and B are more similar in physical and biologic characteristics to each other than they are to type C. Influenza viruses may be spherical or filamentous in shape, and they are of medium size among common viruses of humans. See Animal virus

When a cell is infected by two similar but different viruses of one type, especially type A, various combinations of the original parental viruses may be packaged or assembled into the new progeny; thus, a progeny virus may be a mixture of gene segments from each parental virus and therefore may gain a new characteristic, for example, a new surface protein. This phenomenon is called genetic reassortment, and the frequency with which it occurs and leads to viruses with new features is a significant cause of the constant appearance of new variants of the virus. In the laboratory, reassortment occurs between animal and human strains as well as between human strains. It probably occurs in nature also, and is thought to contribute to the appearance of new strains that infect humans. Generally, if a new variant is sufficiently different from the vaccine currently in use, the vaccine will provide limited or no protection.

The influenza virus has a short incubation period; that is, there is only a period of 1–3 days between infection and symptoms, and this leads to the abrupt development of symptoms that is a hallmark of influenza infections. The virus is typically shed in the throat for 5–7 days. Complete recovery from uncomplicated influenza usually takes several days, and the individual may feel weak and exhausted for a week or more after the major symptoms disappear. The two main complications of influenza are primary influenza virus pneumonia and secondary bacterial pneumonia. Primary influenza pneumonia is relatively infrequent, occurring in less than 1% of cases during an epidemic, although mortality may be 25–30%. The damage to epithelial cells and subsequent loss of the ability to clear particles from the respiratory tract can lead to secondary bacterial pneumonia. This problem commonly occurs in elderly individuals or those with underlying chronic lung disease or similar problems. Influenza-induced pneumonia may cause as many as 20,000 deaths in a typical influenza season. Another complication, known as Reye's syndrome, may follow influenza, and is more common in children. This disease of the brain develops within 2–12 days of a systemic viral infection, and can result in vomiting, liver damage, coma, and sometimes death.

All three types of influenza viruses can cause disease in humans, but there are significant differences in severity of the disease and the range of hosts. In contrast to the large number of animal species infected by type A virus, types B and C are only rarely isolated from animals and infect predominantly humans.

The presence or absence of antibodies is very important in the epidemiology of influenza. In individuals with no immunity, attack rates may reach 70% and severe illness may result. Even low levels of antibody may provide partial protection in an individual and decrease the severity of the illness to only coldlike symptoms.

During an epidemic, one strain of influenza is predominant, but it is not unusual for two or more other strains to be present as minor infections in a population. Outbreaks of influenza occur during cold-weather months in temperate climates, and typically most cases cluster on a period of 1–2 months, in contrast to broader periods of illness with many other respiratory viruses. An increased death rate due to primary pneumonia and bacterial superinfection is common and is one of the ways that public health authorities monitor an epidemic.

Control and prevention of influenza are attempted through the use of drugs and vaccines. Inactivated viral vaccines are used to prevent influenza, although use of attenuated live strains of the virus may better stimulate the cell-mediated immune response and provide higher-quality and longer-lasting immunity. The makeup of the vaccine is modified annually, based upon predictions of the expected prevalent strain for each flu season, but usually contains antigens of two type A viruses and one type B virus. These vaccines take advantage of the natural ability of the viral nucleic acid to reassort and form new strains. The vaccines utilize strains that are not virulent and will replicate at lower temperatures, as found in the nasopharnyx, but not at higher temperatures as found in the lower respiratory tract. The techniques of modern biotechnology are employed to clone copies of parts of the virus or to provide oligonucleotides corresponding to crucial functional areas of the virus, to obtain improved protection and reduced side effects. See Biotechnology, Vaccination

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



grippe, a highly contagious, acute viral disease of the respiratory tract, characterized by a short incubation period (one or two days) and a rapid cyclic course (three to five days).

Influenza affects all age groups in a variety of geographical conditions. The causative agent (discovered in 1933 by the English virologists W. Smith, F. W. Andrewes, and P. Laid-law) is a virus of average size (80–120 millimicrons in diameter) that belongs to the group of myxoviruses. Three antigenic variants of the virus are known: serotypes A and B, which cause mass epidemics that repeat at intervals of two or three years (for serotype A) and three or four years (for serotype B), and type C, which causes sporadic (isolated) illnesses in children of preschool age. A characteristic of virus types A and B is the continuous variability of their antigenic properties, which is accompanied by the periodic emergence of new subtypes that do not produce cross-immunity in affected persons. The most active type A virus causes worldwide (global) epidemics (pandemics). Each pandemic is caused by a new subtype of the virus. Thus the 1918 (“Spanish”) pandemic was caused by the A-0 virus, the 1947 pandemic by A-l, and the 1957 (Singapore) pandemic by A-2. In subsequent years there have been several large epidemics encompassing many countries. The epidemic of 1959 was caused by viruses A2 and B. The epidemic of 1961- 62, which developed in many countries of America, Europe, and Asia, was in some cases caused by the A2 virus and in others by the B. In 1963 a large epidemic in the countries of America was caused by the A2 virus. There was a large A2 epidemic again in 1965. In 1966–67 there were two large epidemics in the USSR—one caused by a group B virus and the other by viruses A2 and B (Hong Kong). The appearance of new antigenic variants is apparently due to the effect of immunological factors on the influenza virus; under the influence of antibodies, the antigens of the influenza virus that are sensitive to them are reduced and replaced by antigens that are resistant to the existing immunity of the population. The morbidity varies from fulminating epidemics, which affect up to 50 percent of the total population, to less massive seasonal outbreaks (winter and early spring). Influenza also occurs in isolated cases between epidemics.

The only source of infection is a person affected with the disease or, more rarely, a healthy carrier of the virus. Infection is spread through air in droplets when a patient coughs, sneezes, or speaks. Moderate inflammation of the mucosae of the nasal passages, pharynx, mouth, larynx, and bronchi are accompanied by a pronounced picture of general intoxication of the body by the endotoxin of the influenza virus. Intoxication causes a sharp diminution of strength and muscular soreness, general breakdown, headache, and increased irritability, There are chills and elevation of temperature, discharges from the mucosae of the nasal passages and throat. conjunctivitis, a dry cough, sneezing, and reddening of the throat and tonsils. The senses of smell and hearing are sometimes impaired. Often complications of bacterial origin develop (inflammation of the middle ear, tonsillitis. laryngitis, tracheitis. bronchitis, pneumonia, meningoencephalitis); dangerous affections of the cardiovascular system are frequent consequences of the general intoxication.

Prevention of influenza and its complications is achieved through a complex of measures, including isolation of the patients at home and a bed regime; mass active immunization of the population with live influenza vaccine from attenuated viruses of serotypes A and B. introduced in atomized form directly to the upper respiratory tract (in the USSR vaccination is conducted annually in the fourth quarter, before the start of the epidemics that usually arise in the first quarter); and treatment and prophylactic use of hyperimmune horse serum during the period of a developing outbreak, introduced as a powdery spray to the upper respiratory tract of the patient and the healthy persons around him (the serum has a therapeutic effect and decreases the patient’s contagiousness, and with simultaneous administration to healthy contacts, tends to render harmless the influenzal breeding ground). Other prospective methods of prevention, by means of interferon and synthetic compounds (amantadine) are at the stage of scientific elaboration.

Specific methods of treating influenza are directed toward alleviating the intoxication by administering immune-donor gamma globulin and suppressing bacterial microflora with the use of antibiotics.


Smorodintsev, A. A., and A. A. Korovin. Gripp. Leningrad, 1961.
Epshtein, F. G. Gripp i grippopodobnye zabolevaniia. [Moscow] 1963.(Bibliography.)


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


An acute virus disease of the respiratory system characterized by headache, muscle pain, fever, and prostration.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


a highly contagious and often epidemic viral disease characterized by fever, prostration, muscular aches and pains, and inflammation of the respiratory passages
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005