My friend is a school nurse, she says that ONE case of measles is considered an epidemic.
From the Merriam-Webster dictionary
1ep•i•dem•ic
Pronunciation:
\ˌe-pə-ˈde-mik\
Function:
adjective
Etymology:
French épidémique, from Middle French, from epidemie, noun, epidemic, from Late Latin epidemia, from Greek epidēmia visit, epidemic, from epidēmos visiting, epidemic, from epi- + dēmos people — more at demagogue
Date:
1603
1:
affecting or tending to affect a disproportionately large number of individuals within a population, community, or region at the same time <typhoid was epidemic>2 a: excessively prevalent b: contagious 4 <epidemic laughter>3: of, relating to, or constituting an epidemic <the practice had reached epidemic proportions>
— ep•i•dem•i•cal \-ˈde-mi-kəl\ adjective
— ep•i•dem•i•cal•ly \-ˈde-mi-k(ə-)lē\ adverb
— ep•i•de•mic•i•ty \-də-ˈmi-sə-tē\ noun