(CNN) Donald Trump has faced scrutiny over his multiple US military deferments during the Vietnam War but many Americans are still questioning what exactly qualifies as a draft exemption -- especially from a medical perspective.

Classification records shared with CNN by the federal government's Selective Service System confirm that Trump received a student deferment and was later found "disqualified for military service" in 1968 after he underwent a physical exam.

No 'habitual drunkenness' or 'masturbation' allowed

Regulations determining which diseases and ailments disqualify a registered man from being drafted for military service can be traced to the Civil War, according to a digital copy of an 1863 regulation manual in the National Library of Medicine that spans 100 pages.

Consider just a few of the many medical conditions that could have disqualified someone from military service, if found to be severe or detrimental, in 1863:

Insanity or mania

Scrofula or constitutional syphilis

Cancer

Habitual drunkenness

Acne rosacea

Masturbation may result in rejection or discharge of service

Minimum stature of 5-foot-3, and possibly maximum height of 6-foot-3

A greater weight than 220 pounds, unless accompanied by corresponding height and muscular sufficiency

Deafness

Hernia and stomach ulcers

Contagious skin diseases

Club feet, splay feet, flat feet

However, "a national bureaucracy for managing conscription did not emerge until after the passage of the Selective Service Act of 1917 -- although even this relied on the contributions of approximately 4,000 local draft boards, which retained the prerogative of granting exemptions," said John Hall , professor of military history at the University of Wisconsin-Madison.

"In 1940, as European and Asian war clouds darkened America's skies, the United States implemented 'peacetime' conscription for the first time -- but the world was already at war," he said. "The real break from American tradition came in 1948 when the United States for the first time employed the draft as a routine element of defense manpower policy, whether or not the United States was at war."

In 1942, the list of medical conditions that could have disqualified someone from military service, if found to be severe or detrimental, looked a little different. Among dozens of other conditions, it included:

Brain tumors

Epilepsy

Sexual perversions

Stammering to such a degree that the registrant is unable to express himself clearly or repeat commands

Psychopathic personalities

Chronic alcoholism and drug addiction

Multiple sclerosis

Cerebrospinal syphilis

"When Richard Nixon took office in 1969, he realized that the draft was undermining rather than sustaining the war effort in Vietnam, so he initiated a transition to an exclusive reliance on volunteers, which culminated in 1973," Hall said. "Partly because of the scarring experience of Vietnam, the United States is very unlikely to resort ever again to the draft."

Modern standards of health

Currently, each branch of the military -- from the Navy to the Air Force -- has its own medical or fitness assessment for applicants.

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"They do have guidelines on what is disqualifying and what is qualifying and, in some cases, what can be waived and what cannot be waived," said Jim Dower, who formerly worked in both Selective Service and the military. He retired in 1994 and now resides in Sarasota, Florida.

"You're psychologically screened, you're physically screened in the normal things you would take a physical for, and your history is taken," Dower said. "If there's any questions, they go out and get consultations for whatever is required."

By then, some of the dozens of medical conditions that could disqualify someone from serving in the US Army, if found to be severe or detrimental, included:

Cleft lip defects

Stomach ulcers and stomach bleeding

Heel spur syndrome and hammertoe result in referral to a medical evaluation board

Current or history of coronary heart disease

Current absence of one or both testicles

Plantar flexion of the foot must meet 30 degrees

Women below 58 inches or over 80 inches tall do not meet standards

Men below 60 inches or over 80 inches tall do not meet standards

"Although there has not been a draft in over 40 years, men 18 [years old] are still required to register with the Selective Service System . It's a law and civic duty," said Matthew Tittmann, a spokesman for the agency.

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"At 26, they become too old to register, but failure to register can carry lifelong consequences, and non-registrants risk being disqualified from access to federal college loans and grants, job training programs, all federal jobs and many state and municipal jobs," he added. "All documented and undocumented immigrants must register, as well. Otherwise they risk losing the aforementioned benefits and could delay their citizenship process."