Huck pretends that he and his father have a disease so the bounty hunters do not come closer.
When Huck runs into the men who are looking for slaves, he has a decision to make. At this point, remember, he is still fighting with his conscience. He does not know what to say when he is asked if the man on the raft with him is white or black. He still believes that it is wrong to be on the run with a slave, because that is what society has told him. He has also been told that it is wrong to lie. So he has a moment’s wrestling with his conscience.
I didn't answer up prompt. I tried to, but the words wouldn't come. I tried for a second or two to brace up and out with it, but I warn't man enough—hadn't the spunk of a rabbit. I see I was weakening … (Ch. 16)
Huck thinks that he is acting immoral when he hides Jim, acting out of cowardice or friendship instead of doing what is right. He is putting his own needs and his own moral code above society’s. It does not occur to him yet that society is the one that is wrong.
Huck uses a bit of reverse psychology on the men, begging them to come closer so that they stay away. He never says exactly what is wrong with his “father” and lets their own imaginations do the trick. Pretty soon, not only do they not only not want to investigate, but they do not want to be anywhere near Huck and Jim!
“Keep away, boy—keep to looard. Confound it, I just expect the wind has blowed it to us. Your pap's got the smallpox, and you know it precious well. Why didn't you come out and say so? Do you want to spread it all over?” (Ch. 16)
Now that Huck has convinced them that he and his father have smallpox, the men cannot get away fast enough. Huck really starts to get into the act, saying that everyone they encounter has abandoned them. This tugs on the men’s heartstrings, and far from being suspicious, they are sympathetic. They tell Huck that he is twenty miles from a town, and give him money.
This incident demonstrates Huck’s ingenuity and quick thinking. It also shows his independence. His lifestyle has made him immune to much of society’s indoctrination. Tom Sawyer, for example, would never consider actually freeing a slave moral. He thinks it is all a game. Huck on the other hand independently comes to the conclusion that society is wrong. Slavery is immoral. He decides that if he is going to go to Hell for feeling as he does, that is fine with him.