The Israelites did not enjoy the hygiene and sanitation methods that are used today. Various conditions cause the body to emit unsanitary discharges. Persons afflicted with those conditions were declared to be unclean and commanded to undergo certain procedures after the discharge ended in order to be clean again. God provided those procedures in order to promote health and safety among His people.
Any person or object coming in contact with an unclean person or an object that the unclean person had touched was declared to be unclean. One’s cleansing was determined by the severity of his discharge or whether only contact with uncleanness had been made.