influenza

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An acute contagious viral infection of humans, characterized by inflammation of the respiratory tract and by fever, chills, muscular pain, and prostration.
• noun Any of various viral infections of domestic or wild animals, generally characterized by fever and respiratory involvement.

from The Century Dictionary.

• noun An epidemic catarrh of an aggravated kind, attended with serious febrile symptoms and rapid prostration.
• noun A prevailing influence; an epidemic.
• noun Influenza is an acute infectious disease of which the most prominent symptoms are fever, general prostration, pains in various parts of the body, and inflammatory processes attacking the serous or mucous membranes, the lungs, or the nerves. The onset is usually abrupt with headache, backache, fever, and loss of strength. According to the organs most affected in its further progress, the disease is said to be of the respiratory, nervous, or gastro-intestinal form. Influenza is noteworthy for the rapidity with which an epidemic sweeps over entire countries and even from one continent to another, and for the large proportion of the population attacked when it is prevalent. The disease is caused by a small, non-motile bacillus which occurs in great numbers in the nasal and bronchial secretions of the patients. Influenza has a low death rate, but its effect on the general health is often severe and lasting, and many grave sequelæ are possible. One attack does not protect against a second. The ordinary influenza or ‘grippe,’ though in some ways simulating true epidemic influenza, is a different and much milder disease.
• noun An infectious specific fever of horses, asses, and mules, characterized by alterations of the blood, great depression of the vital forces, and inflammatory complications, especially of the lungs, intestines, and brain. It usually assumes an epizootic form.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Med.) An epidemic viral infectious disease characterized by acute nasal catarrh, or by inflammation of the throat or the bronchi, and usually accompanied by fever and general weakness; also called grippe. It is caused by several forms of RNA virus which mutate readily and thereby render vaccines prepared against older forms ineffective, often requiring a new form of vaccine for each new outbreak.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun pathology An acute contagious disease of the upper airways and lungs, caused by a virus, which rapidly spreads around the world in seasonal epidemics.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an acute febrile highly contagious viral disease