Communication by contact or by what looks like contact. The contagion of vice, of heresy.
Communication of an illness by immediate or immediate contact. This disease is caught by contagion.
Before Pasteur, the phenomenon of contagion was relatively well known (observed), but poorly explained.
Contagiousness (called “contagion” at the beginning of the 20th century) being the potential for transmission of a disease from person to person. In terms of risk management, the worst case scenario is that of a highly pathogenic and highly contagious microbe.