Capable of being contracted (in various senses).
Able to infect others.
Memorable and invoking excitement or interest.
Spreading quickly from one person to another.
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).