California Code, Health and Safety Code - HSC § 101185

(a) A local health department serving one or more counties that shall provide services to all cities whose population is less than 50,000 in addition to the unincorporated territory of the county or counties.

(b) A county health department that does not serve all of the cities of less than 50,000 population, but that has the provisional approval of the department, in accordance with Section 101225.

(c) The health department of a city of 50,000 or greater population, except that the governing body of the city by resolution may declare its intention to be included under the jurisdiction of the county health department, as provided by existing statutes.

(d) The local health department of any county that had under its jurisdiction on September 19, 1947, a population in excess of 1,000,000, or the local health department of any city and county.