You can read INA 212 here. That provision is "Classes of Aliens Ineligible for Visas or Admission.-Except as otherwise provided in this Act, aliens who are inadmissible under the following paragraphs are ineligible to receive visas and ineligible to be admitted to the United States:... Health-related grounds. ... In general. - Any alien ..who is determined (in accordance with regulations prescribed by the Secretary of Health and Human Services in consultation with the Attorney General) -(I) to have a physical or mental disorder and behavior associated with the disorder that may pose, or has posed, a threat to the property, safety, or welfare of the alien or others, or
(II) to have had a physical or mental disorder and a history of behavior associated with the disorder, which behavior has posed a threat to the property, safety, or welfare of the alien or others and which behavior is likely to recur or to lead to other harmful behavior,....
is inadmissible"