The quality or degree of being contagious; the tendency of a disease or influence to spread from person to person.
From 'contagious' plus the abstract noun suffix '-ity' (from Latin). This technical term appears mainly in medical and epidemiological literature to describe the transmissibility of diseases.
In epidemiology, 'contagiosity' is measured precisely—the R-value (reproduction number) tells scientists exactly how contagious something is, turning an old poetic word into hard mathematical data.
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