"study of epidemics, science of epidemic diseases," 1850, from Greek epidemios, literally "among the people, of one's countrymen at home" (see epidemic) + -logy. Related: Epidemiological; epidemiologist.
c. 1600, "common to or affecting a whole people," originally and usually, though not etymologically, in reference to diseases, from French épidémique, from épidemié "an epidemic disease," from Medieval Latin epidemia, from Greek epidemia "a stay in a place; prevalence of an epidemic disease" (especially the plague), from epi "among, upon" (see epi-) + dēmos "people, district" (see demotic). Also see -ic.
word-forming element meaning "a speaking, discourse, treatise, doctrine, theory, science," from Greek -logia (often via French -logie or Medieval Latin -logia), from -log-, combining form of legein "to speak, tell;" thus, "the character or deportment of one who speaks or treats of (a certain subject);" from PIE root *leg- (1) "to collect, gather," with derivatives meaning "to speak (to 'pick out words')." Often via Medieval Latin -logia, French -logie. In philology "love of learning; love of words or discourse," apology, doxology, analogy, trilogy, etc., Greek logos "word, speech, statement, discourse" is directly concerned.
<a href="https://www.etymonline.com/word/epidemiology">Etymology of epidemiology by etymonline</a>
Harper, D. (n.d.). Etymology of epidemiology. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/epidemiology