Neville wakes up in his house. He sees that the front door is wide open: Ruth is gone. He finds a note from Ruth, explaining, “I want to save you if I can.” The note explains that Ruth was sent to spy on Neville: she wore makeup to make herself look tan. Ruth volunteered to spy on Neville because Neville killed Ruth’s husband, but Ruth developed “feelings” for Neville during the course of their time together. The note goes on to explain that “we” are going to set up a society, “slowly but surely.”
Ruth’s letter provides some quick, much-needed exposition. As in many science fiction novels of the era, the romantic connection between Ruth and Neville isn’t portrayed very convincingly. Furthermore, it seems like a potentially sexist assumption that Ruth would forgive and fall for her husband’s killer after only one night of seduction.
Ruth’s note goes on to explain that she and her friends are infected with the germ, but have learned how to survive with it. They can stay out in the sun for short periods of time, and they take special pills that help them tolerate the germ. Ruth explains that she doesn’t blame Neville for killing some of her friends with wooden stakes, as she knows that he was just trying to survive. She adds that she’s left one of her pills; these pills both nourish the vampiris germ and also keep it from multiplying and destroying the host. She encourages Neville to take the pill and then get as far from Los Angeles as he can. The note ends, “I wasn’t spying on you. I was loving you.”
For most of the novel, Neville has operated on the assumption that vampires—and those who’ve contracted the vampire germ—can’t move around in the sunlight. Now, he learns that there are some vampire-like creatures who’ve learned how to control the vampiris bacterium with special pills. This further suggests that, for a long time now, Neville has been killing off sentient, civilized vampire-creatures, an act that Ruth finds morally despicable (hence her question at the end of Chapter 17).
Neville finds a small pill next to the note. He feels confused about what’s just happened: he remembers that he and Ruth “had embraced, they had …” In the midst of his confusion, however, Neville remembers a sentence he’s read again and again in his textbooks, a sentence he’s never fully appreciated until now: “Bacteria can mutate.”
The implication of this passage is that, over the last three years, some vampires—Ruth included—have mutated from feral, dangerous creatures into sentient, intelligent beings, with their own society. (A further implication is that Neville accidentally has intercourse with a vampire.)