Vaccine
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Influenza A is a viral disease of global dimension, presenting with high morbidity and mortality in annual epidemics, and in pandemics which are of infrequent occurrence but which have very high attack rates. Influenza probes reveal a continuing battle for survival between host and parasite in which the host population updates the specificity of its pool of humoral immunity by contact with and response to infection with the most recent viruses which possess altered antigenic specificity in their hemagglutinin (HA) ligand. HA ligand binds the virus to the cell to bring about infection. ⋯ Pandemics are believed, conventionally, to be derived solely by rare events in which wild viruses of man acquire a new HA ligand of avian origin. There might be an alternative possibility involving a periodicity in selective control by the host population itself, in its receptivity or rejection at a particular time of particular reassortant viruses which might be created more frequently in nature than we are presently aware. This hypothesis, though remote, provides a different way to view and to probe the enigma of pandemic influenza.