- (pathology, of an illness) 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.
- More infectious diseases like the flu are usually less potent.
- (pathology, of a person) Able to infect others.
- Despite feeling better, the patient is still infectious.
- (of feelings and behaviour) Spreading quickly from one person to another.
- Her enthusiasm for work can be really infectious.
- (informal) Memorable and invoking excitement or interest.
- Pop music is more infectious than elevator music.
The terms infectious, communicable, transmissible/transmittable, and contagious, as well as spreadable and catching, overlap on a semantic field and are often loosely used synonymously in their broad senses, although they are differentiable by narrower senses, as follows:
- The word infectious describes any disease or condition that is caused by an infectious agent (such as a bacterium, virus, parasite, or prion), including ones with person-to-person transmission/spread and ones without person-to-person transmission/spread. It is thus hypernymous to the following terms.
- The words communicable and transmissible/transmittable, as well as spreadable and catching (which are informal), describe the large subset of infectious diseases in which person-to-person transmission/spread (communication) can occur, including ones that are readily/easily spread and ones that are not readily/easily spread. They are thus hypernymous to the following term.
- The word contagious describes only those infectious diseases that are readily/easily spread, to the degree that preventing their spread is quite difficult unless a population is highly vaccinated against them (examples include measles and diphtheria).
See Usage notes.