Leviticus 14:8-9 is just one of the many verses that you’ll find in the long list of verses on apologist web sites that are supposed to prove the bible has prophetic knowledge about science. In this case, it’s that the bible preaches prevention and/or curing disease long before science knew about it. Like all the other items found in these lists, it’s highly embellished and taken completely out of context.
As Quazar points out, God doesn’t tell the leper to wash their body until after they’re already healed of the disease, long after it would do any good. That is, if it would do any good, which it wouldn’t. While you can wash away bacteria on your skin, no amount of scrubbing will cure leprosy. Next, God tells them that they should wait eight days before washing, which is the exact opposite of what you’d want to do to prevent any diseases that actually could be cured by washing. Third, God demands that the person shave off all the hair on their heads, including their beard and eyebrows, which will accomplish absolutely nothing. In fact, this demonstrates how clueless the bible really is. Remember, the bible warns against hair turning white, and then tells you to cut it off. This implies that the hair somehow became diseased, and by removing it, it helps make you clean. Incidentally, the bible doesn’t say anything about shaving off body hair. So, if your leprosy turns your arm hair white, I guess you’re just supposed to leave it! It would make more sense for God to command the shaving of any hair turned white by leprosy. Well, it wouldn’t make sense to our modern understanding of leprosy, but at least it would be consistent with their ignorance.
The point is, had the authors of these lists actually looked at the context of the verses, they would understand how dishonest it is to present them as prophetic knowledge. This is quote mining, pure and simple.
One other nitpick, the bible words these two verses strangely. God says, the person to be cleaned is to wash his clothes, shave his hair, and wash his body, and then he is clean and may enter the camp after he remains outside for seven days. In the next verse, God awkwardly says, but on the seventh day, shave all his hair, wash his clothes, and wash his body. Bizarre repetition is typical for the bible, but he could have at least kept the same order of events in both lists.