A single-cell parasite, Toxoplasma gondii, is perhaps best-known for its connection to cats. The parasite can move from its feline host to humans, most commonly through contact with cat feces. And while the parasite typically only causes mild infection (people may have flulike symptoms), in people with weakened immune systems, infections can cause serious problems, from seizures to severe lung problems.

But a T. gondii infection can also have some downright bizarre effects. Over the years, the parasite has not only surprised and stumped researchers, but has also led to new insights into how human behaviors work.