infectious

Short definition from Wiktionary

Adjective

1. 1
   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).
2. 2
   Able to infect others; capable of transmitting pathogens.
3. 3
   Spreading quickly from one person to another.
4. 4
   Memorable and invoking excitement or interest.