Earth Abides by George R. Stewart

In this profound ecological fable, a mysterious plague has destroyed the vast majority of the human race. Isherwood Williams, one of the few survivors, returns from a wilderness field trip to discover that civilization has vanished during his absence. Eventually he returns to San Francisco and encounters a female survivor who becomes his wife. Around them and their children a small community develops, living like their pioneer ancestors, but rebuilding civilization is beyond their resources, and gradually they return to a simpler way of life.