McCandless thinks that the wild potato seeds made him sick. He wrote in his book "EXTREMLY WEAK. FAULT OF POT. SEED...” .
At first Krakauer believed that McCandless mistook the wild potato plant for another poisonous plant and that is what killed him. However, after doing more research, Krakauer learns that some plant seeds are toxic even though the actual plant is not. Wild potatoes have toxic seeds and he believes that McCandless suffered from swainsonine poisoning which kept his body from absorbing the nutrients it needed from other foods.