Definition of "infectious"

(pathology, of an illness) Caused by an agent that enters the host's body (such as a bacterium, virus, parasite, or prion); often, also, transmitted among hosts via any of various routes (for example, contact, droplet-borne, airborne, waterborne, foodborne, fomite-borne, or bloodborne).

(pathology, of a person) Able to infect others.

(of feelings and behaviour) Spreading quickly from one person to another.  examples

(informal) Memorable and invoking excitement or interest.  examples