1. a disease-causing agent or infectious material that spreads from one organism to another
Usage: archaic or formal; largely replaced by modern terms like 'pathogen' or 'infectious agent'
2. the transmission or spread of disease or infection from one person or organism to another
Usage: archaic or formal; historical medical usage

Examples

• Medieval physicians debated whether contagium was a physical substance or a miasma.
• The contagium of plague spread rapidly through the crowded city.
• Early germ theory proposed that contagium could be transmitted through direct contact.
• Scientists studied the contagium responsible for the epidemic.
• The doctor warned that contagium from the infected patient could spread to others in close proximity.