adjective 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).
Cancer is usually not infectious.
adjective Able to infect others; capable of transmitting pathogens.
Despite feeling better, the patient is still infectious.
adjective Spreading quickly from one person to another.
infectious smile
adjective Memorable and invoking excitement or interest.