Definition: the most common type of influenza. These strains have a high propensity for antigenic change resulting in mutations, partly because they can infect various animals where dual infections can occur, giving rise to new hybrid strains. The infections occur in epidemics, which may occur every 2–3 years and may vary in size and severity; perhaps the most important of the three types of influenza (A, B, and C).
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Examples: glitazone, GI cocktail, etc.