Tarrou pontificates in Albert Camus’ The Plague:

Each of us has the plague within him; no one, no one on earth is free from it. And I know too that we must keep endless watch on ourselves lest in a careless moment we breathe in someone’s face and fasten the infection on him. What’s natural is the microbe. All the rest – health, integrity, purity (if you like) – is a product of the human will, of a vigilance that must never falter. The good man, the man who infects hardly anyone, is the man who has the fewest lapses of attention. And it needs tremendous will power, a never-ending tension of the mind, to avoid such lapses.