Contagion.
Synonym:
infection.
Infection is frequently confused with contagion, even by medical men. The best usage now limits contagion to diseases that are transmitted by contact with the diseased person, either directly by touch or indirectly by use of the same articles, by breath, effluvia, etc. Infection is applied to diseases produced by no known or definable influence of one person upon another, but where common climatic, malarious, or other wide-spread conditions are believed to be chiefly instrumental.