Simon has sick spells multiple times throughout Lord of the Flies, and while Golding never directly says what ails Simon, he provides enough clues to the reader to conclude that Simon has some type of sickness that causes him to have fits. Jack alludes to this when the boys first arrive on the island:
Then one of the boys flopped on his face in the sand and the line broke up. [...] "He's always throwing a faint," said Merridew. "He did in Gib.; and Addis; and at matins over the precentor" (20).
Simon's fainting spells suggest that he may have epilepsy, a chronic disorder of the nervous system that can result in seizures. Simon seems to have his seizures most often when he is extremely fatigued or too hot. When he meets the Lord of the Flies in the dense, steamy jungle, the hot air seems to close around Simon, and then the young boy loses consciousness:
Simon was inside the mouth. He fell down and lost consciousness. (144)
The last line of chapter eight, not only shows Simon losing the battle of wills with the Lord of the Flies, but also suggests that he is overcome by physical exhaustion.