Contagious & Infectious

Contagious (pronounced kuhn-tey-juhs)

(1) Capable of being transmitted by bodily contact with an infected person or object.

(2) Carrying or spreading a contagious disease; bearing contagion, as a person or animal with an infectious disease that is contagious.

(3) Tending to spread from person to person.

1350–1400: From the Middle English, from the Old French contagieus (which endures in Modern French as contagieux), from the Late Latin contāgiōsus, the construct being contāgi(ō) (contagion) (a touching, contact," often in a bad sense, "a contact with something physically or morally unclean, contagion") + -ōsus (from the Old Latin -ōsos from -ōnt-to-s  from -o-wont-to-s, the last form being a combination of two primitive Indo-European suffixes (-went- & -wont- and -to-); the suffix -ōsus was added to a noun to form an adjective indicating an abundance of that noun, much as -εις (-eis) operated in Ancient Greek).  The Latin contingere (to touch) came from an assimilated form of com (with, together) + tangere (to touch) from the primitive Indo-European tag- (to touch, handle).  Originally a technical word purely used in medicine, the figurative sense in which it could be applied to anything apt to spread from one to another (rumors etc) dates from the 1650s.  Contagious is an adjective, contagion, contagionist, contagionism, contagiousness & contagosity are nouns and contagiously is an adverb; the most common noun plural is contagions.

Infectious (pronounced in-fek-shuhs)

(1) Communicable by infection, as from one person to another or from one part of the body to another.

(2) Causing or communicating infection (of a disease) caused by pathogenic microorganism or agent, such as bacteria, viruses, or protozoa.

(3) Tending to spread from one to another.

(4) In international law, capable of contaminating with illegality; exposing to seizure or forfeiture.

(5) Diseased (wholly obsolete).

1535–1545: A compound from Middle English, the construct being infect From Middle French infect, from Latin infectivus & infectus (from the Proto-Italic enfaktos (the construct being in- (not) + factus perfect passive participle of faciō (do, make)), perfect passive participle of inficere & inficiō (dye, taint) + -ious (an alternative spelling of -ous, from the Middle English -ous, borrowed from the Old French -ous and -eux, from the Latin -ōsus (full, full of) and a doublet of -ose in unstressed position; the suffix was used to form adjectives from nouns, to denote possession or presence of a quality in any degree, commonly in abundance).  The sense of "catching diseases, having the quality of spreading from person to person, communicable by infection" dates from the 1540s which by the early seventeenth century had spread to emotions, actions et al; earlier in the same sense were infectious, common by the late fifteenth century and infective from a hundred-odd years earlier.  The most novel adaptation of the word was the sense of "captivating", noted first in the 1650s.  Disinfectant (agent used for destroying the germs of infectious diseases) dates from 1837 from the French désinfectant (1816), noun use of present participle of désinfecter, or else from the adjective in English (by 1827) in the sense of "serving to disinfect".  Infectious is an adjective, infection, infectionist, infectionism, infector & infectiousness are nouns and infectiously is an adverb; the most common noun plural is infections.  

Sort of interchangeable

Increasing the risk of contagion: Lindsay Lohan sneezing.

Except for specialists such as virologists or epidemiologists, contagious and infectious can probably be used interchangeably although, when used in the figurative sense, many style guides suggest contagious should be used if referring to something undesirable whereas infectious should be preferred if speaking of the pleasantly irresistible quality of something.  There’s no etymological basis for this; it’s just a convention of use.  Something contagious is a thing which can be transmitted from one living being to another through direct or indirect contact.  Although infectious is also used to describe the process, it has a slightly different meaning in that it refers to diseases caused by infectious agents (such as or SARS-CoV-2 which causes COVID-19) not normally present in the body.  While the notion of contagiousness dates at least from Antiquity, the idea of infectious diseases is more modern, arising only after the publication of the germ theory of disease, not proposed until the late nineteenth century.  Contagious and infectious are also used to refer to people who have communicable diseases at a stage at which transmission to others is likely.

Reducing the risk of contagion: Lindsay Lohan in facemask (although the twin one-way, non-return valves of these masks limit their effectiveness in reducing the risk of infecting others).

For those in the relevant professions however, the difference between the two is significant.  “Infectious” is a description a disease-causing agent’s (typically a virus, bacterium or parasite) ability to enter, survive, and multiply in a host organism; by definition any infectious disease is caused by the presence and activity of such agents.  The best-known infectious diseases include the various strains of influenza (commonly clipped to “the flu”), tuberculosis (TB), malaria, hepatitis, AIDS (HIV the agent) and of course COVID-19 (Sars-COV-2 the agent).  “Contagious” refers to the ability of a disease to spread from one host (such as a bacterium, human or other animal) to another through a variety of vectors including (1) direct contact (shaking hands, kissing, sexual contact (these sometimes sequential)), (2) indirect contact (such as touching a door knob contaminated with the infectious agent using one’s hand which then introduces the agent to the system via the eyes, nose or mouth), (3) airborne transmission (usually by breathing in droplets when an infected person in close vicinity coughs or sneezes) or (4) through a third party (such as animal scratch or bite).

A contagious disease is a sub-set of infectious diseases which can (some more easily than others) be transmitted from one host to another.  Examples of contagious diseases include measles, chickenpox (and other –pox types), COVID-19 and most commonly, the endemic common cold.  While all contagious diseases are infectious, not all infectious diseases are necessarily contagious; tetanus is an infectious disease caused by the bacterium Clostridium tetani, but it is not contagious from host to host.