The word leper was historically used to refer to someone who suffered from leprosy, a bacterial illness that affects the nerves, skin, and respiratory tract. Because leprosy was thought to be highly contagious, the word leper also came to be used more generally to mean "an outcast" or "a person to be avoided."
Although it was originally used a medical term — the Greek root is lepros, "scaly" — today the word leper is generally considered to be offensive. In the Middle Ages, when there was no known treatment for leprosy, people with the disease were often quarantined in areas known as "leper colonies." These days, leper is more commonly used in a general way to refer to someone who is shunned by others: "Her friends treated her like a leper because they mistakenly thought she had been spreading rumors about them."