1 an infectious disease can be passed easily from one person to another, especially through the air they breathe:
Flu is highly infectious. * (figurative) infectious laughter
2 [notusuallybeforenoun] if a person or an animal is infectious, they have a disease that can be spread to others:
I'm still infectious.
compareCONTAGIOUS
infectiously adverb:
to laugh infectiously
