A material or agent that transmits disease; the contagious matter believed to spread infection from person to person.
From Latin 'contagium' meaning 'touching together' and 'infection.' This term was used extensively in medical literature from the 16th-18th centuries and remains in formal medical and historical discourse.
The word 'contagium' is what medieval scientists called what we now know to be viruses, bacteria, and other pathogens—they couldn't see them, but they theorized their existence centuries before microscopes proved them right.
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