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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