I remember we had a guy bum-rushed out of South Pole station because he was found to have hepatitis. Of course, there are qualified doctors who will make the decisions in this regard, considering contingencies such as the risk either that the person under medication may become incapicitated due either to the lack of it, or its ineffectiveness given an arduous environment; and the possible risk to other site personnel due the the infectuous nature of the individual’s condition.
There’s always the famous question “how do you feel” as a part of the interview and qualification.