An acute infectious respiratory disease, caused by Influenza viruses, which are in the family Orthomyxoviridae, in which the inhaled virus attacks the respiratory epithelial cells of those susceptible and produces a catarrhal inflammation; characterized by sudden onset, chills, fever of short duration (3–4 days), severe prostration, headache, muscle aches, and a cough that usually is dry and may be followed by secondary bacterial infections that can last up to 10 days. The disease commonly occurs in epidemics, sometimes in pandemics, which develop quickly and spread rapidly; the mortality rate is usually low, but may rise in patients with secondary bacterial pneumonia, particularly in old people and those with underlying debilitating diseases; strain-specific immunity develops, but mutations in the virus are frequent, and such immunity usually does not affect antigenically different strains. Syn: flu, grippe.
Influenza viruses are divided on the basis of antigenic structure into three types. Influenza A virus is principally responsible for epidemics; subtypes of influenza A virus affect birds, horses, and swine as well as human beings. Incidence of influenza B is lower and epidemics are less likely to occur with this virus, for which animal reservoirs are apparently of little importance. Influenza C infection is typically mild or subclinical. The annual mortality of influenza in the U.S. is believed to exceed 50,000, more than 90% of these deaths occurring in people 65 years of age or older. Influenza deaths have increased substantially in the past 20 years, in part because of the aging of the population. At least 30 pandemics of influenza have occurred since 1580. The influenza A pandemic of 1918–1920 (“Spanish flu”) caused more than 20 million deaths worldwide, 500,000 of them in the U.S. Less devastating pandemics occurred in 1957 (“Asian flu”) and 1968 (“Hong Kong flu”). Influenza is highly contagious. The virus is transmitted from person to person by direct contact and by airborne droplets of respiratory secretion expelled through coughing and sneezing. Incidence is highest in late fall, winter, and early spring. Active immunization with noninfective vaccines containing hemagglutinin (H) and neuraminidase (N) antigens of currently prevalent strains has reduced the extent and severity of epidemics and has provided protection to vulnerable populations such as the elderly. Vaccines are especially recommended for people older than 50 and those with certain chronic conditions, including diabetes mellitus, immune deficiency, impairment of renal function, and cardiac and pulmonary disease. Immunity arising from either natural infection or vaccination confers protection only against certain strains of virus. Antigenic drift results from the gradual accumulation of new epitopes on viral H and N molecules, whereas antigenic shifts are caused by mutations in the genes that encode these molecules. A new strain probably emerges as a human pathogen when that strain is transmitted to human beings from animal hosts. Influenza cannot be diagnosed with certainty or differentiated from other acute febrile syndromes on clinical evidence alone. Diagnosis can only be confirmed by detection of viral antigen in nasal secretions by direct immunofluorescence or by a rising titer of antibody to influenzal hemagglutinin. The antiviral drugs amantadine and rimantadine (effective only against influenza A) and the neuraminidase inhibitors oseltamivir and zanamivir can prevent clinical illness when taken prophylactically during an outbreak or epidemic and can reduce the severity and duration of symptoms (average reduction in duration with all agents, one day) when administered within 24–48 hours after the onset of illness. An international network for influenza surveillance was established by the World Health Organization in 1948. Now consisting of 110 centers in 83 countries, the network monitors influenza activity worldwide, facilitates rapid identification of viral strains, and provides information used in determining the composition of influenza vaccines.
Reference: Stedman's Medical Dictionary