CONTAGION, in physic, the communicating a disease from one body to another. In some diseases it is only effected by an immediate contact or touch, as the venom of the pox; in others, it is conveyed by infected cloaths, as the itch; and in others, it is transmitted through the air at a considerable distance, by means of steams or effluvia expiring from the sick, as in the plague and other pestilential disorders, in which case the air is said to be contagious.