Infectious means able to be spread by infection, like a disease that spreads from one person to another. You'll be uncomfortable shaking Uncle Marvin's hand if you're worried that warts are infectious.

Something that's infectious spreads easily, like infectious cold germs that might be left on a door handle — anyone who touches it might get sick. If your mother tells you the cut on your knee looks infectious, you'd better go to the doctor. Some infectious things are good — like infectious laughter that gets the whole class giggling, or your friend's infectious good mood that makes you forget your cares.

Definitions of infectious
  1. adjective
    of or relating to infection
    infectious hospital”
    infectious disease”
  2. adjective
    caused by infection or capable of causing infection
    synonyms: infective
    infected, septic
    containing or resulting from disease-causing organisms
  3. adjective
    easily spread
    “fear is exceedingly infectious
    catching, communicable, contagious, contractable, transmissible, transmittable
    (of disease) capable of being transmitted by infection
    easily diffused or spread as from one person to another
    contaminating, corrupting
    that infects or taints
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    not infectious
    noncommunicable, noncontagious, nontransmissible
    (of disease) not capable of being passed on
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