surgery - visa hindrance?

good day!

i would just like to inquire if undergoing an operation negatively affects the granting of a visa for the states? i understand that all applicants are required to undergo a medical exam, but i've been hearing a lot of talk such as, "so and so can't enetr the states because she had this operation, etc...". is there any truth to that? let's say, the applicant underwent a major operation or something..