The King James Version translates it as 'leprosy.' That is wrong. The NIV translates it as, variously, both 'infectious skin disease' if it is on a person and 'mildew' if it is on a thing. That is not terribly accurate either, although it is better than "leprosy." The Hebrew word used in Leviticus 13 and other places comes from the root meaning 'scourge.' And that is what is refers to in a generic sense. This would include leprosy, infectious skin diseases, molds, mildews, etc. One reason the interpretation of leprosy was given by the KJV translators was undoubtedly because leprosy does cause eruption in multiple gross and deep sores on the body. The NIV chose 'infectious skin disease' where the skin is concerned because the uncleanness is declared when it is spreading on the person or more than skin deep. In the second case, this could even include some types of skin cancer. Where buildings, cloths, and leathers are concerned in the last part of the chapter, I spent some time on the net checking "green mildew", "red mildew", "green mold" and "red mold." From what I could see, the green varieties are scourges for a number of food plants and can also attack leather in humid areas. It is always damaging. The red mold or mildew was a surprise, though. As nasty as it can seem, it is evidently from strains of this that some of our antibiotics come! It is also cultivated in the Orient for use with rice and some beverages. It is hard to know exactly what is meant by the last part of chapter 13, however, because the term 'mildew' and 'leprosy' are, in the first case, too general and, in the second, too impossible. Buildings don't get leprosy! Nor can leprosy be caught from a moldy or mildewy building. All of this said, chapter 13 is a virtual diagnostic chart of skin diseases -- it's just that they don't name them individually as we do. God simply declared the signs for knowing whether they were infectious or not and whether they made a person ceremonially unclean or not. It is the sort of chapter we tend to skip over unless we have sores on our skin! Nevertheless, it is good to know this chapter is there and to be able to refer to the information in it occasionally.