Leviticus 15:8

If he... spit upon him. — Spitting in the face of a person was, and still is, commonly resorted to among Oriental nations as an expression of insult and contempt (Numbers 12:14; Deuteronomy 25:9; Isaiah 1:6; Job 30:10; Matthew 26:67, &c.). Suffering from the disease here discussed, the patient would naturally be more irritable, and hence be liable to give vent more frequently to this mode of expressing his wrath. Now, any person upon whom he happened to heap this indignity became defiled by the spittle in the same manner, and had to go through the same purification, as he who chanced to touch his garments, or as the physician who had to examine him.