The use of the word leprosy in many English translations is probably erroneous. If you look at a scholarly annotated bible like the Oxford Annotated Bible New Revised Standard Version (NRSV) you'll see that the original word whether Hebrew from the Old Testament or the Aramaic of the New Testament did not exclusively mean leprosy but likely covered a wide range of skin disorders. So the short answer is that leprosy in the Bible is not always leprosy but rather a bunch of diseases that did not have separate names.

I question I would pose is when did the translation of those words as leprosy begin?