Toxoplasma gondii is an obligate, intracellular, parasitic protozoan (kingdom: Chromista) that causes the disease toxoplasmosis.
Found worldwide, T. gondii is capable of infecting virtually all warm-blooded animals. In humans, it is one of the most common parasites; serological studies estimate that up to a third of the global population has been exposed to and may be chronically infected with T. gondii, although infection rates differ significantly from country to country.. Although mild, flu-like symptoms occasionally occur during the first few weeks following exposure, infection with T. gondii generally produces no symptoms in healthy human adults. However, in infants, HIV/AIDS patients, and others with weakened immunity, infection can cause serious and occasionally fatal illness (toxoplasmosis).
Toxoplasma gondii in "Something Going Around"
During their first conversation, Indira Patel and Stan discussed, among other things, Toxoplasma, specifically how is can survive in nearly any host, and can even reproduce asexually without ill effects to healthy humans. Stan, who'd lost two friends with AIDS to toxoplasmosis, was familiar with those facts. However, he was surprised to learn that Toxoplasma can only reproduce sexually inside cats. Indira further explained that mice infected with Toxoplasma grow less fearful of cats, making them easier prey, and allowing the Toxoplasma to move into its new host.
When Indira confirmed that Toxoplasma did influence the behavior of even healthy humans to a minor degree, 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.
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 thought they 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 Toxoplasma made mice less wary of cats.