What's the "een" in Halloween?
1590s, "fact of having mastery," from Middle French prévalence (15c.), from Late Latin praevalentia, from praevalens, present participle of praevalere (see prevalent). Meaning "condition of being widespread or general" is from 1713.
prevalence prev·a·lence (prěv'ə-ləns)
The total number of cases of a disease in a given population at a specific time.