Why doesn't the slave hunters get Jim in the book huckleberry Finn?

Huck Finn rows out in a canoe to a boat that has two slave hunters. When Huck Finn is approached by the slave hunters, he says that his father, mother, and sister, Mary Ann, all have small pox. However, this is just a tall tale that Huck makes up, because he doesn't want the slave hunters to go near the raft which Jim is on. Therefore, the slave hunters are afraid to go near the raft and don't capture Jim.