4. For the purposes of section 3, a teacher has a physical or mental disability if that teacher suffers from a severe and prolonged medical condition.
A medical condition is severe if by reason thereof the teacher is totally incapable of performing the work required by his or her position for an extended period of time.
A medical condition is prolonged if it is expected to last indefinitely, that is, if it is unlikely that a cure is possible in the current state of medical knowledge.