n 1 a  :  an acute highly contagious virus disease that is caused by various strains of an orthomyxovirus belonging to three major types and that is characterized by sudden onset, fever, prostration, severe aches and pains, and progressive inflammation of the respiratory mucous membrane - often used with the letter A, B,or Cto denote disease caused by a strain of virus of one of the three major types <the type of A that has been widespread in the United States this winter -H. M. Schmeck, Jr.>   b  :  any human respiratory infection of undetermined cause - not used technically  2  :  any of numerous febrile usu. virus diseases of domestic animals (as shipping fever of horses and swine influenza) marked by respiratory symptoms, inflammation of mucous membranes, and often systemic involvement in*flu*en*zal adj 
 
 
 
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