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It's a long story that dates back all the way to the 1790s it looks like. (Just learning about it myself). But it took on the military nature for civilian doctors around this time:
>In 1870 a bill was passed to centralized and reorganize the Marine Hospital Service, with its headquarters in Washington DC under the position of supervising surgeon. The act also raised the hospital tax from twenty cents to forty cents until 1884. John Maynard Woodworth was the first supervising surgeon. He studied natural history and medicine in Chicago, graduating from Rush Medical College in 1862 and joining the Union Army. He quickly transformed the Marine Hospital Fund into a true system, the Marine Hospital Service. Woodworth borrowed the military model which was the only extant of appointment to public office in the 1870 and adopted it to the service. He instituted examinations for applicants and implemented required uniforms for physicians. He created a cadre of mobile, career service physicians assigned to various marine hospitals. In 1873, his title was changed to Supervising Surgeon General. His innovations were but one aspect of period of portentous development in the nascent discipline of public health.
There's more to it. Can be read up on here.
https://careers.publichealth.iu.edu/blog/2018/10/12/a-brief-history-of-the-public-health-service/