infectious

English

infectious (comparative more infectious, superlative most infectious)

(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, typically of a person) Able to infect others; capable of transmitting pathogens.
- 1981 December 1, Sue Hyde, quoting Paul Black, “AIDS Update: Little To Cheer About”, in Gay Community News, volume 12, number 20, page 3:
  I doubt that [saliva] is very infectious ... I don't think this is a normal route of transmission.
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).