A medical probe for traversing a tract in the body of a human or animal. One medical probe includes an elongate elastically deformable member including a helically wound element, and a sheath having an inner surface which contacts an outer surface of the elastically deformable member. The probe is elastically deformable, for traversing the tract, by virtue of the elastically deformable member. The probe may include a treatment element, which can be used to perform a procedure in the body of a patient. An assembly includes a probe and a treatment element. The medical probe has a particular use in the treatment of a fistula, in which the probe takes the form of a fistula probe adapted to traverse a fistula tract.