An unidentified parasite inhabiting human hosts arrived in a university community. While this parasite could survive in humans, it needed to leave the host in order to enter the next stage of its life-cycle. Thus, it altered the behavior of its hosts, causing them to carelessly step into the path of oncoming traffic. The injuries caused when the human host was killed gave the parasite a sufficient means of exiting the body and finding a new host.
A languages professor named Stan had seen the aftermath of one such accident. Some weeks later, he met parasite ecologist Indira Patel. The two discussed their respective careers. After Indira explained how certain parasites, such as malaria, flatworms, and Toxoplasma gondii could influence the behavior of their respective hosts, Stan wondered if there could be a parasite that could live in people, but required another host for sexual reproduction, and whether such a parasite could alter human behavior to allow for the required change in host. Indira allowed that, unlike some other animals, human behavior is difficult to influence, but would not say that it was impossible.
Weeks later, Indira was killed when she inexplicably stepped into the path of a car. Stan, who'd been passing by, realized who the victim was. He also made the intuitive leap as to the nature of the mystery parasite. Stan was left worried that he might be a host as well.