The board of county health center trustees shall not enter into contracts for the private practice of medicine, nor shall any of its personnel practice medicine nor dispense drugs, vaccines or serums for personal gain, nor shall its facilities be used for such purpose in any way except as it may be necessary and agreed upon between the board and county commission or commissions for the care of the indigent for whom the commission or commissions may be responsible, or except in furtherance of diagnostic and communicable disease control programs.