Except as provided in subsections (b) and (e) of this section, original appointments to the Regular Corps may be made only in the warrant officer (W–1), chief warrant officer (W–2), chief warrant officer (W–3), chief warrant officer (W–4), junior assistant, assistant, and senior assistant grades and original appointments to a grade above junior assistant shall be made only after passage of an examination, given in accordance with regulations of the President, in one or more of the several branches of medicine, dentistry, hygiene, sanitary engineering, pharmacy, psychology, nursing, or related scientific specialties in the field of public health.

Original appointments to the Ready Reserve Corps may be made to any grade up to and including the director grade but only after passage of an examination given in accordance with regulations of the President. Reserve commissions shall be for an indefinite period and may be terminated at any time, as the President may direct.

No individual who has attained the age of forty-four shall be appointed to the Regular Corps, or called to active duty in the Ready Reserve Corps for a period in excess of one year, unless (A) he has had a number of years of active service (as defined in section 212(d) of this title) equal to the number of years by which his age exceeds forty-four, or (B) the Surgeon General determines that he possesses exceptional qualifications, not readily available elsewhere in the Commissioned Corps of the Public Health Service, for the performance of special duties with the Service, or (C) in the case of an officer of the Ready Reserve Corps, the Commissioned Corps of the Service has been declared by the President to be a military service.