Picture the Israelites, wandering the desert, and having to go outside the camp for 7 days every little while until they were cleansed of whatever mildew or dead person they had been in contact with. Now, don't forget that the Israelite camp was made of some 3 million people, possibly much more, I don't recall the numbers. That's a big freakin camp. And even if the nation of Israel was divied up into it's 12 tribes, you'd still have to get yourself outside a camp of like 10,000 people. Imagine this continuous spread of people as they make their way outside, and then back in, and so on and so forth. No wonder they spent 40 years in the desert.