(a) The jurisdiction of any county or district health department shall extend over all unincorporated areas and over all municipal corporations within the territorial limits of the county or the counties comprising the district, but not over the territory of any municipal corporation which at the time of the establishment of the county or district health department had a population in excess of twenty-five thousand (25,000) according to the most recent federal census and which maintains its own health department and employs a supervising health officer; however, any such municipal corporation not otherwise within the jurisdiction of a department, by the passage of an ordinance by three-fourths (3/4) of the elected members of its city council, board of trustees, or other governing body, and by agreement with the county or district board of health approved by the State Board of Health, may merge its department with the county or district health department.

(b) All local boards of health existing within the county or district, except those of any municipal corporation which at the time of the establishment of the county or district health department had a population in excess of twenty-five thousand (25,000) according to the most recent federal census and which maintains its own health department, employs a supervising health officer, and does not elect to merge its health activities with the county or district health department shall be dissolved upon the organization of a county or district health department under the provisions of this act, or upon the acceptance of a county into a district health department.

(c) All cities of the first class under twenty-five thousand (25,000) in population incorporated in any county or district health department may contribute an equitable share of the annual financial cost of the health department.