Definition: 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.
[It. influence (of planets or stars), fr. L. influentia, fr. in-fluo, to flow in]