Let us consider that chapter. This necessitates the ability to dispense knowledge. It is the duty of elders to present the correct food in proper amounts at the right time to assure the full growth of every member. They are to be able to provide what is lacking in spiritual development. They are not required to be apt to preach, which, in the New Testament sense, has to do with announcing of the Good News to aliens.
Teaching is instruction in systematic fashion.
It has nothing to do with acquiring knowledge, but of conveying it to others. Regardless of how much a man may know about the Bible, if he does not have the ability to instruct others, he cannot qualify as a bishop. The shepherds should be able to supply every intellectual need of the flock in the spiritual realm. The diseased are those who are ravaged by protracted maladies or who suffer from a chronic condition which weakens them. The shepherd must be skilled in diagnosis.