The truth is, Sam had what they call in South Carolina country fever, a high type of malarial fever, which stupefies and benumbs its victim almost as soon as it attacks him.

The dews in the far South, especially in the fall, are so heavy that the water will drip and even stream off the foliage of the trees all night, and Sam had been drenched every night during both his journeys, having no fire by which to warm himself or dry his clothes.
Even without this drenching the poisonous exhalations of the swamps and woods would doubtless have given him the fever, and as it was he had it very severely.

He laid down again almost under his horse's feet and fell into a sort of stupor.

He knew that his fever required treatment, and that it would rapidly sap his strength, and the thought came to him: What if he should die there and never get back to the tree fortress?
He was too sick to care for himself, but the thought of little Judie haunted his dreams, and he was seized with a semi-delirious impulse to remount his horse and ride straight away to the hiding-place in which he had left her, regardless of Indians, and of everything else.

He dreamed a dozen times that he was doing this, and finally, when morning came, he forgot all about the danger of travelling by daylight, and mounting his horse in a confused, half-delirious way, rode straight out of the woods towards the open country, which he had hitherto so carefully avoided..