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.

Not more than 10 per centum of the original appointments to the Regular Corps authorized to be made during any fiscal year may be made to grades above that of senior assistant, but no such appointment (other than an appointment under section 205 of this title) may be made to a grade above that of director. For the purpose of this subsection the number of original appointments authorized to be made during a fiscal year shall be (1) the excess of the number of officers of the Regular Corps authorized by the appropriation Act or Acts for such year over the number of officers on active duty in the Regular Corps on the first day of such year, plus (2) the number of such officers of the Regular Corps who, during such fiscal year, have been or will be retired upon attainment of age sixty-four or have for any other reason ceased to be on active duty. In determining the number of appointments authorized by this subsection an appointment shall be deemed to be made in the fiscal year in which the nomination is transmitted by the President to the Senate.

No person shall be appointed pursuant to this subsection unless he meets standards established in accordance with regulations of the President.

For purposes of basic pay and for purposes of promotion, any person appointed under subsection (a) to the grade of senior assistant in the Regular Corps, and any person appointed under subsection (b), shall, except as provided in paragraphs (2) and (3) of this subsection, be considered as having had on the date of appointment the following length of service: Three years if appointed to the senior assistant grade, ten years if appointed to the full grade, seventeen years if appointed to the senior grade, and eighteen years if appointed to the director grade.

For purposes of basic pay, any person appointed under subsection (a) to the grade of senior assistant in the Regular Corps, and any person appointed under subsection (b), shall, in lieu of the credit provided in paragraph (1) of this subsection, be credited with the service for which he is entitled to credit under any other provision of law if such service exceeds that to which he would be entitled under such paragraph.

For purposes of promotion, any person originally appointed in the Regular Corps to the senior assistant grade or above who has had active service in the Ready Reserve Corps shall be considered as having had on the date of appointment the length of service provided for in paragraph (1) of this subsection, plus whichever of the following is greater: (A) The excess of his total active service in the Ready Reserve Corps (above the grade of junior assistant) over the length of service provided in such paragraph, to the extent that such excess is on account of service in the Ready Reserve Corps in or above the grade to which he is appointed in the Regular Corps or (B) his active service in the same or any higher grade in the Ready Reserve Corps after the first day on which, under regulations in effect on the date of his appointment to the Regular Corps, he would have had the training and experience necessary for such appointment.

For purposes of promotion, any person whose original appointment is to the assistant grade in the Regular Corps shall be considered as having had on the date of appointment service equal to his total active service in the Ready Reserve Corps in and above the assistant grade.

A former officer of the Regular Corps may, if application for appointment is made within two years after the date of the termination of his prior commission in the Regular Corps, be reappointed to the Regular Corps without examination, except as the Surgeon General may otherwise prescribe, and without regard to the numerical limitations of subsection (b).

Reappointments pursuant to this subsection may be made to the permanent grade held by the former officer at the time of the termination of his prior commission, or to the next higher grade if such officer meets the eligibility requirements prescribed by regulation for original appointment to such higher grade. For purposes of pay, promotion, and seniority in grade, such reappointed officer shall receive the credits for service to which he would be entitled if such appointment were an original appointment, but in no event less than the credits he held at the time his prior commission was terminated, except that if such officer is reappointed to the next higher grade he shall receive no credit for seniority in grade.

No former officer shall be reappointed pursuant to this subsection unless he shall meet such standards as the Secretary may prescribe.