Almost all applicants for Permanent Residence must take and pass a medical exam administered by a U.S. Civil Surgeon, which is just a physician who has been approved by USCIS for these purposes. You must ensure that the physician you use is approved by USCIS. The exam consists principally of tests for tuberculosis, syphilis, and gonorrhea, blood and urine tests, vaccinations where necessary, and a few other screenings. Results are given in a sealed envelope, but you should insist on receiving an open copy as well so that you or your attorney can review for errors in filling out the form that may cause substantial delays with your case. The forms are poorly designed, and so errors by the medical staff are not uncommon. Sometimes, conditions irrelevant to medical eligibility such as pregnancy are erroneously listed as serious health conditions that imply a need for long-term institutionalization or incapacity. It is best to catch these before submitting the results to USCIS.
Medical exams conducted outside the United States for those who are consular processing abroad are even more problematic. Staff at these facilities may aggressively question applicants about past alcohol or substance abuse, even in the far past, attempting to elicit a "confession" that can render an applicant temporarily or permanently inadmissible. Some reports state that even an admission to casual use of alcohol has led to the requirement of lengthy alcohol "counseling" before a visa can be issued. There is also an extensive inquiry into any tattoos that applicant may have as staff attempt to determine whether they represent gang affiliation. If you have any potential issues concerning alcohol, controlled substances, or tattoos you should speak in depth with an attorney to assess whether you can safely navigate this process and how best to handle any questions truthfully without creating unnecessary problems.
If you have objections to vaccines, discuss this with your attorney before advancing too far in this process. Although the law provides for religious exemptions they are difficult to obtain and require proof of a sincerely held religious belief. Objections not based on religion carry no weight at all. Pregnant applicants should also discuss the implications with their attorney to see how vaccinations and chest x-rays will be handled and whether it is best to proceed with the exam while pregnant or to wait.