adjective

• (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.
• (informal) Memorable and invoking excitement or interest.