influenza or flu, acute, highly contagious disease caused by a virus; 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 a particular strain, 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 350 cases of A (H5N1) influenza have been identified in humans, largely in Asia; roughly 60% of the cases have been fatal.

avian influenza: see influenza.

or Spanish influenza epidemic

Most severe influenza outbreak of the 20th century. It apparently started as a fairly mild strain in a U.S. army camp in early March 1918. Troops sent to fight in World War I spread the virus to western Europe. Outbreaks occurred in nearly every inhabited part of the world, spreading from ports to cities along transportation routes. Pneumonia often developed quickly and killed within two days. Among the most deadly epidemics in history, it left an estimated 25 million dead; unusually, half the deaths were among 20- to 40-year-olds.

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or flu or grippe

Acute viral infection of the upper or lower respiratory tract. Influenza viruses A (the most common), B, and C produce similar symptoms, but infection with or vaccination against one does not give immunity against the others. Chills, fatigue, and muscle aches begin abruptly. The temperature soon reaches 38–40 °C (101–104 °F). Head, muscle, abdominal, and joint aches may be accompanied by sore throat. Recovery starts in three to four days, and respiratory symptoms become more prominent. Bed rest, high fluid intake, and aspirin or other antifever drugs are standard treatment. Influenza A tends to occur in wavelike annual pandemics. Mortality is usually low, but in rare outbreaks (see influenza epidemic of 1918–19) it reaches immense proportions. Most deaths result from pneumonia or bronchitis.

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or avian influenza

viral respiratory disease, mainly of birds including poultry and waterbirds but also transmissible to humans. Symptoms in humans include fever, sore throat, cough, headache, and muscle aches. Severe infections can result in life-threatening complications such as pneumonia and acute respiratory illness. The first known human cases occurred in Hong Kong in 1997, resulting in six deaths. Deadly outbreaks among poultry in several countries in eastern and central Asia between 2003 and mid-2005 were accompanied by more than 100 human cases, about half of them fatal. The causative agents are virus subtypes related to the human influenza type A viruses, the most virulent and contagious being the H5N1 subtype. A specific protective vaccine for this virus remains to be developed. Studies suggest that some antiviral drugs that work against human influenza may be effective in treating bird flu in humans.

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