mortality (n.)

mid-14c., mortalite, "condition of being subject to death or the necessity of dying," from Old French mortalite "massacre, slaughter; fatal illness; poverty; destruction" (12c.) and directly from Latin mortalitem (nominative mortalitas) "state of being mortal; subjection to death," from mortalis "subject to death, mortal," from PIE root *mer- "to rub away, harm" (also "to die" and forming words referring to death and to beings subject to death).

Meaning "widespread death, numerousness of deaths; plague" is from c. 1400; meaning "number of deaths from some cause or in a given period" is from 1640s, later especially in proportion to population.