What superstitions do huck and Jim believe?

He's mostly uneducated, and a lot of uneducated people are superstitious. Answer: You've got to take into account the time and setting of the story. In the mid-1800's most people in the poor southern states had a significant amount of superstition in their lives - diseases were the result of toads, dog's baying at the moon were harbingers of death and warts were cured with rituals in the dead of night. This is how Twain remembers his boyhood and uses these memories to make the story seem alive.