Clean and Unclean: Impurity of Persons

People could become impure in various ways, for example by:

Swellings, a rash or marks on the skin could be indicators of tzaraat. This skin disease resulted in a state of impurity, which was only over when the skin affliction disappeared or the affected person had died. The person with tzaraat had to be separated from the community.
If the affliction was healed, there followed a “declaration of purity” by the priest. After that the healed person had to bring a purification offering, wash his clothes, cut off his hair and finally wash his body. After that he could rejoin the community. The rules that applied to tzaraat are mainly found in Leviticus 13–14.

Certain fluids excreted by the body, namely (menstrual) blood, semen and fluids after giving birth, made a person unclean. The period of impurity that ensued depended on the sort of excretion. Such flows were probably seen as defiling because they were connected with reproduction. Strong, incomprehensible forces were ascribed to reproduction.
Rules on this sort of impurity are to be found in particular in Leviticus 12 and Leviticus 15.

Impurity was also caused by touching a corpse, a human bone or a grave (Numbers 19). As dead people had to be buried, it was not possible to avoid such impurity entirely. But to become pure again one had to undergo extensive purification rituals.

As impurity is contagious, according to the Torah, a person could also become unclean by touching an unclean person or object (see for instance Leviticus 5:3; Numbers 19:16). But in such cases people did not need to be set outside the community.