incident vs contagion

incident and contagion both are nouns.

Word	Noun	Adjective	Verb	Adverb
incident	Yes	Yes	No	No
contagion	Yes	No	No	No

As nouns, contagion is a hyponym of incident; that is, contagion is a word with a more specific, narrower meaning than incident:

Other hyponyms of incident include cause celebre, infection, transmission, scene, sideshow.

incident (noun)	contagion (noun)
a single distinct event	the communication of an attitude or emotional state among a number of people
a public disturbance	an incident in which an infectious disease is transmitted
	any disease easily transmitted by contact

incident (adjective)	contagion (adjective)
(sometimes followed by `to') minor or casual or subordinate in significance or nature or occurring as a chance concomitant or consequence
falling or striking of light rays on something