Contagion

n.
  1. (Med.) The transmission of a disease from one person to another, by direct or indirect contact.
  2. That which serves as a medium or agency to transmit disease; a virus produced by, or exhalation proceeding from, a diseased person, and capable of reproducing the disease.
  3. The act or means of communicating any influence to the mind or heart; as, the contagion of enthusiasm.
  4. Venom; poison.

The term has been applied by some to the action of miasmata arising from dead animal or vegetable matter, bogs, fens, etc., but in this sense it is now abandoned.