question:  Why did Charles think he was the last person alive?
context: There should be an epitaph for every man, big or little, but a really grand and special one for Loner Charlie. New York lay quietly below him. To any man, WHO HAPPENED TO BE THE LAST MAN ON EARTH! I just don't know—" A paroxysm of sobbing shook his body. Buildings were taller, men were healthier, most of the problems of humanity seemed licked, or nearly so. The largest cotton crop ever was forecast and rumors from Mexico had it that no one had died from scorpion bite in several weeks. A month later the meat animals, the birds and the household pets began dropping as rapidly as the flies which had dropped earlier. Within a year it was obvious to everyone that man was the only animal left on earth. The panic which had begun with the death of the animals was quieted somewhat by the fact that humans seemed immune to the pandemic. But the lakes full of dead fish caused a great stink and residents along the coasts began to move inland. Within six months, half of the world's population was gone. Less than a month ago no more than a few thousand people remained in New York. And now.... "I've got to find out," Charles told himself. Before the plague the Bureau of Vital Statistics had been one of man's crowning achievements. Once a year the Bureau issued The Index, an exact accounting of Earth's four billion inhabitants. In all of his life, Charles had never once glanced at The Index. Reaching the gigantic building, Charles pushed aside the body of a young man and walked into the main foyer. Only once, before the plague, had he seen the interior of this room. And again at the age of 10 each child returned to the Bureau for a recheck. It was for this latter recording that Charles had come to the Bureau some twenty-two years before and a friendly guard had let him peep briefly into the computer room.
output:  He had some sort of immunity
options:  His sickness was taking longer  He had some sort of immunity  He was the reason for the plague  He was meant for greater things