Being ritually unclean was a normal part of Jewish life. Things that would make a person unclean included coming into contact with a dead animal, certain moulds, a woman’s monthly cycle, childbirth, entering the house of a Gentile, eating certain foods, leprosy, or touching a dead person. Being ritually unclean was not a sin, but it did prevent that person from participating in certain ceremonies.
It was the responsibility of the unclean person to return to ritual purity as soon as possible. There were different states of ritual impurity that required different treatments. Most impurities simply required the person to separate themselves for a time and then wash themselves in water while some states of uncleanness required the presentation of an atoning sacrifice.