Mange is caused by tiny parasites. It forces the animal to suffer from painful itching, loss of hair and sores. Sores are caused by scratching and biting from the pet to relieve the incessant irritation.
A responsible owner would take the pet to a veterinarian and realize that it is treatable with medicated baths. A bad owner would ignore the problem and allow it to persist.
Some forms of mange are also contagious to people.