By way of general background about physician education and training, medical doctors in the United States receive a degree of either M.D. (medical doctor) or D.O. (doctor of osteopathic medicine). All are physicians who may be licensed to practice medicine and surgery (and all are “medical doctors”).
In order to be licensed after graduating with an M.D. or a D.O. degree, a physician must complete a minimal amount of training in an American Board of Medical Specialties (ABMS)- or American Osteopathic Association (AOA)-approved program. That is called a residency; during training, such physicians are called residents.
Within the ABMS are individual boards (e.g., the American Board of Internal Medicine (ABIM), the American Board of Neurological Surgery (ABNS), the American Board of Pediatrics (ABP)) that credential the training programs, administer the board certification examinations, and credential physicians as “board certified.”
In parallel, within the American Osteopathic Association (AOA) are individual boards (e.g., the American Osteopathic Board of Obstetrics & Gynecology, the American Osteopathic Board of Internal Medicine, American Osteopathic Board of Surgery) that credential the training programs, administer the board certification examinations, and credential physicians as “board certified.”
Only after completion of an ABMS-approved residency is a physician “board eligible” (i.e., allowed to take the board certification examination) in the corresponding ABMS board. In parallel, only after completion of an AOA-approved residency is a physician “board eligible” (i.e., allowed to take the board certification examination) in the corresponding AOA board.
A D.O. is allowed into either an AOA- or an ABMS-approved training program. (Although not relevant to this case, historically, M.D.s were not allowed into an AOA training program. This may soon or may already have changed.) There are more ABMS residencies and fellowships nationwide, so many osteopathic physicians enroll in ABMS programs for training. Only physicians trained in ABMS-approved programs (both M.D.s and D.O.s) may be boarded by an ABMS board. Only physicians trained in AOA-approved programs may be boarded by an AOA board.
Many residencies, including radiology, dermatology, anesthesiology and psychiatry, require the first year of residency to be in basic medicine (e.g., in internal medicine or pediatrics). The purpose is to assure that these physicians have basic medical knowledge (felt to be necessary) before beginning specialized training.
After completion of residency (e.g., in internal medicine, pediatrics, general surgery, orthopaedic surgery, anesthesiology), a physician may choose to complete an optional ABMS- or AOA-approved program. That is called a fellowship; such physicians are called fellows.
Fellowship programs last one or more years, after which the physician is then board eligible for the applicable “sub” board certification examination, if one exists. (Not all fellowship programs have a corresponding “sub” board certification examination.)
Infectious disease is an example of a fellowship field within internal medicine; indeed, it does have a “sub” certification examination available to those who complete an ABMS- or AOA-approved program. Interventional radiology is an example of a fellowship field within radiology; indeed, it does have a “sub” certification examination available to those who complete an ABMS-approved program.