Flatworm

Flatworms, or Platyhelminthes, Plathelminthes, or platyhelminths are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrates. Over half of all known flatworm species are parasitic. While historically dangerous to humans in developing countries, various species have become a threat to the developed countries.

Flatworm in "Something Going Around"
During their first conversation, Indira Patel told Stan about a type of flatworm that inhabits the intestinal tracts of of sticklebacks, causing the sticklebacks to change their behavior in search of food, making them easier prey for herons. While the flatworm can survive in the sticklebacks, they need to be in herons for the next stage of their life cycle.

This example in part led Stan to wonder 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 if it was impossible.

Some weeks later, Stan had confirmation when Indira joined a growing number of pedestrians who inexplicably stepped into the path of an oncoming vehicle and was killed. Stan were hosts to some sort of parasite, and that their careless actions were actually the parasite trying to escape the humans and find a host suitable for reproduction, much as flatworms made sticklebacks less wary of herons.