Mo. Rev. Stat. § 334.150
It is not intended by sections 334.010 to 334.140 to prohibit isolated or occasional gratuitous service to and treatment of the afflicted, and sections 334.010 to 334.140 shall not apply to physicians and surgeons commissioned as officers of the Armed Forces of the United States or of the public health services of the United States while in the performance of their official duties, nor to any licensed practitioner of medicine and surgery in a border state attending the sick in this state, if he does not maintain an office or appointed place to meet patients or receive calls within the limits of this state, and if he complies with the statutes of Missouri and the rules and regulations of the department of social services relating to the reports of births, deaths and contagious diseases; and sections 334.010 to 334.140 shall not apply to Christian Science practitioners who endeavor to cure or prevent disease or suffering exclusively by spiritual means or prayer, so long as quarantine regulations relating to contagious diseases are not infringed upon; but no provision of this section shall be construed or held in any way to interfere with the enforcement of the rules and regulations adopted and approved by the department of health and senior services or any municipality under the laws of this state for the control of communicable or contagious diseases.
§ 334.150, RSMo