Leviticus chapter 13, part 2

* Now that we are done with skin ailments, we move on to mildew found on articles made of wool, linen, or  leather. If the mildew is green or reddish, it is spreading mildew. It must be shown to the priest. The article is to be isolated for seven days. If the mildew is still spreading, the article is definitely unclean and must be burned. If the mildew does not spread, the article must be washed and isolated for another seven days. If it looks the same, even though it has not spread, it is unclean and must be burnt. If the mildew has faded, it can be cut out and the article can be used. However, if the mildew reappears in that article and spreads, it must be burnt. If the mildew is gone after washing, an article is clean.

*Today we know that mildew is caused by a fungus that grows in dampness. If materials are kept dry, by regular exposure to circulating air, dry heat, or strong sunlight, they will not mildew. Also, vinegar works well to kill fungus. These are all things ancient people could have used instead of the wait and see method favored by the priests. This tells me they didn't know or care about prevention, just the ambiguous status of clean or unclean.