Epidemiology of influenza.

The epidemiological pattern of influenza over the past century is outlined; its vagaries are ascribed to the plasticity of the influenza virus. In recent years it has been possible to carry out antigenic studies revealing variations in the strains of virus A, the agent in most major outbreaks. New antigenic variants may have a greater chance to emerge after periods of latency, since the exposed population will have less specific resistance to them. Work on the causation and spread of epidemics is reviewed; the conclusion these studies suggest is that two factors are involved-the existence of a "basic virus", as yet undetectable until activated, and responsible for multicentric outbreaks; and the element of "spread", which under favourable conditions carries the infection to susceptible groups, and results in extensive epidemics.