Ancient Israel was a society and a culture ruled by strict laws governing what was clean and what was unclean. These distinctions were fundamental to what it meant to be an Israelite; you could say they lay at the very heart of Israel’s identity as a nation.
A couple of examples. If you had leprosy, you were unclean. What’s more, you had to announce this fact to everyone around you by shouting “Unclean, unclean!”. (One can only imagine how devastating that must have been to a person’s self-esteem.) And you had to live “outside the camp”: you were effectively excommunicated from society.
Now, since leprosy was a life-threatening disease, you might think the Israelites were somewhat justified in strictly separating themselves from lepers. But what about this: equally strict rules applied to women when they had their period, or when they had a discharge outside of their period. They were unclean, and anything they touched became unclean. They could only be made clean again by observing strict rules and offering sacrifices at the temple through the mediation of a priest. (No doubt it wasn’t easy being a woman in that culture. But just so the men don’t feel left out, similar rules applied to any man who had a nocturnal emission.)
There were also, of course, unclean foods and unclean animals. And perhaps the ultimate in uncleanness was a dead body, which no one, not even a priest, was allowed to touch.
Furthermore, uncleanness was catching. The prophet Haggai tells us that if anyone became unclean by touching a dead body and then went on to touch any kind of food, the food he touched would become unclean as a result. Interestingly, he also tells us that the reverse was not true: bringing unconsecrated food into contact with consecrated food did not make the unconsecrated food clean. One might say that uncleanness was more powerful than cleanness.
So many rules determining who was clean and who was unclean, who was in and who was out, who was acceptable and who was unacceptable. To be unclean was to be unholy, to be effectively excluded from God’s people. Uncleanness was like a disease that could be caught and spread.