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"study of epidemics," 1873, from Greek epidemios "epidemic" (see epidemic) + -logy. Related: Epidemiological; epidemiologist.
epidemiology ep·i·de·mi·ol·o·gy (ěp'ĭ-dē'mē-ŏl'ə-jē, -děm'ē-)n. The branch of medicine that deals with the study of the causes, distribution, and control of disease in populations.