Paul doesn’t use the word “cancer” in this chapter (1 Cor 5). That word was only coined by teh Greek doctor Hippocrates in about 400BCm so teh Old Testament authors used a different metaphor instead. God chose yeast, which like cancer spreads rapidly and secretly, because the Hebrews knew its power to ferment and destroy food left un protected. The Lord command Aaron and the priests, “Do not offer the blood of a sacrifice to me along with anything containing yeast” and he ordained the seven day feast of Unleavened Bread each year after the Passover. Jesus had applied this principle in Mark 8:15 when he warned his followers to “Watch out for the yeast of teh Pharisees and that of Herod” and Paul had applied it earlier in Galatians 5:9 when he warned that “A Little yeast works through the whole batch of dough”. Yeast, cancer: it didn’t really matter which metaphor he used. Sexual immorality within teh church at Corinth would spread, infect and destroy the whole body unless they dealt with it radically.
From the book "Straight to the Heart of 1 & 2 Corinthians: 60 Bite-sized Insights (The Straight to the Heart Series)"