Is it permissible for a doctor to mix with a plague patient in order to treat him?

There is nothing to indicate the permissibility of a physician mingling with a plague-stricken person. The fundamental principle is not to enter a land afflicted by plague and not to leave it in flight. Also, "Let not one who is ill come to one who is healthy." However, it has been established that for every ailment there is a cure, and seeking treatment is permissible. If a physician has protective measures that guard him from infection, then there is no harm in treating a plague-stricken person. But if protective measures are not available, the fundamental principle is to keep away from a person with a contagious disease.