What was most commendable about John Snow's work? "For herd immunity to occur, which of the following must be true?" A) The disease must occur in a single host species B) Populations must avoid contact C) A vaccine must be developed D) A large proportion of the popazation must be immune E) Populations must interact constantly "A, B, and C only" C and D only "A, D, and E only" All of the above None of the above.