Chris McCandless resumed his solitary life at his camp after finding the Teklanika uncrossable. He caught enough game to subsist for a month, and apparently spent that time hunting, gathering, and reading. At the end of that time, he "made the mistake that pulled him down". On July 30, his journal entry reads indicates that he was "extremly weak" and in "great jeopardy". He faulted "pot.seed" for his predicament.
There have been many conjectures as to what exactly caused Chris's "precipitous decline". It is possible that he ingested potato seeds he had brought in with him, which become toxic after they have begun to sprout. More probably, he might have mistakenly eaten a toxic plant that closely resembles an edible one, or have been poisoned by the mold growing on seeds that were ordinarily safe to consume. Whatever the case, Chris McCandless died of starvation on or around August 18, "112 days after he'd walked into the wild". The last book he read before dying was Doctor Zhivago; he may have had an epiphany before he died, because he noted in the book as "most vexing of all" the statement "happiness only real when shared".
One of his last acts before crawling into his sleeping bag and slipping into unconsciousness was to take a picture of himself. In it, he looks emaciated but happy, and "at peace, serene as a monk gone to God" (Chapter 18).