On July 8, 1992, McCandless returns to the bus. He resumes hunting small game and gathering edible berries and wild potatoes, but he is burning more calories than he consumes. He reads Tolstoy's The Death of Ivan Ilyich and finishes Boris Pasternak's Doctor Zhivago, writing "HAPPINESS ONLY REAL WHEN SHARED." — a striking sentiment from someone so relentlessly compelled toward solitude.
On July 30, McCandless makes an ominous entry in his journal: "EXTREMELY WEAK, FAULT OF POT. SEED. MUCH TROUBLE JUST TO STAND UP. STARVING. GREAT JEOPARDY." Krakauer points out that until this journal entry, nothing suggests that McCandless is in danger of starving to death. Though hungry, he is otherwise in good health. Less than one month later, he will be dead. How?
Wayne Westerberg suggests that McCandless ate some potato seeds he bought in South Dakota; potato seeds can become toxic once they have sprouted. But he would have needed to eat many pounds of these seeds, and he doesn't seem to have done so. There is, however, a wild potato that McCandless may have foraged for — and confused with the similar-looking, and toxic, wild sweet pea.
The author imagines a hungry McCandless mistaking one plant for the other and becoming incapacitated. Already worn down by a subsistence diet, his body wasn't able to stave off the emetic effects of the plant, which ultimately killed him. As time goes on, however, Krakauer begins to doubt this hypothesis.
Some four years after McCandless's death, Krakauer finally discovers that a toxic mold can grow on legumes. "I had an epiphany," he writes. "It wasn't the seeds of the wild potato that had done McCandless in; he was probably killed instead by mold that had been growing on those seeds."
Krakauer describes the effects of poisoning by the mold: "The body is prevented from turning what it eats into a source of usable energy. If you ingest too much . . . you are bound to starve, no matter how much food you put into your stomach."
On August 5, McCandless notes in his journal that he has spent 100 days in the wild. Then he writes "BUT IN WEAKEST CONDITION OF LIFE. DEATH LOOMS AS SERIOUS THREAT." Again the author points out McCandless's folly in not having a topographical map: only six miles south of the bus was a Park Service cabin, equipped with first-aid supplies, bedding, and emergency food — a three-hour walk away. Krakauer, however, notes that even the existence of this cabin would not have saved McCandless, since the cabin had been recently vandalized, and anything edible within had been exposed to wild animals and the weather.
McCandless writes his final journal entry on August 12. Barely a week later, he tears a page out of Western author Louis L'Amour's memoir, Education of a Wandering Man, that quotes a poem by Robinson Jeffers, "Wise Men in Their Bad Hours." On the back of this page, McCandless writes, "I HAVE HAD A HAPPY LIFE AND THANK THE LORD. GOODBYE AND MAY GOD BLESS ALL."
Did McCandless finally come to forgive his family, as evinced by the "HAPPINESS ONLY REAL WHEN SHARED" inscription he wrote toward the end of his life? Perhaps — but note that in all of his writings, there is nothing that explicitly reaches out to his parents or his sister, Carine. McCandless never acknowledges them, even to say goodbye.
Note, too, that Krakauer's theory on McCandless's death, that it was caused by mold on wild potato seeds, is just that: a theory. It is not definitive. To some degree it is beside the point anyway, since one could argue that it wasn't so much starvation that killed McCandless as arrogance and shortsightedness.