Charles Ellicott Commentary Leviticus 15:8

"And if he that hath the issue spit upon him that is clean, then he shall wash his clothes, and bathe himself in water, and be unclean until the even." — Leviticus 15:8 (ASV)

If he ... spit upon him. —Spitting in a person’s face was, and still is, commonly practiced in Eastern nations as an expression of insult and contempt (Numbers 12:14; Deuteronomy 25:9; Isaiah 1:6; Job 30:10; Matthew 26:67, and other similar passages). Suffering from the disease discussed here, the patient would naturally be more irritable and therefore more likely to express his wrath in this way more often. Now, any person on whom he happened to heap this indignity became defiled by the spittle in the same way, and had to go through the same purification, as someone who happened to touch his garments, or as the physician who had to examine him.