Christopher has a recurring dream in which a virus has killed almost everyone on earth. (We think we've seen that movie…)
Sometimes it's a daydream, but tonight is one of the nights he dreams it while he's asleep.
The virus isn't a normal virus, but one that's transmitted by "the meaning of something an infected person says and the meaning of what they do with their faces when they say it" (229.2). (Okay, we take it back about the movie… this is way different.)
Of course, Christopher would have no danger of catching such a virus.
Eventually (in his dream), the only people left on earth are the people like him. Each of them likes to be alone, and Christopher can walk around freely and do whatever he wants.