A bad infection can make you delirious. But usually the mind clears once the germs are gone. Usually.

Not so when researchers recently injected mice with Toxoplasma gondii. This parasitic protozoan only gets amorous inside the gut of a cat. The resulting fertilized eggs wind up in a litter box. Their challenge is to grow up and get back inside a feline. The solution: get eaten by a mouse that gets eaten by a cat.

The expelled eggs wait until they become a meal for a mouse. The parasites then move to the mouse’s brain. Where they stop the rodent from fearing the smell of cat urine. The delirious mouse approaches Fluffy and, voila, the parasite finds its way home.

In past trials, mice almost always died of this kind of infection. This study used weaker strains of Toxo and the mice recovered—but they stayed blasé about the cat smell.  [Wendy Marie Ingram et al., Mice Infected with Low-Virulence Strains of Toxoplasma gondii Lose Their Innate Aversion to Cat Urine, Even after Extensive Parasite Clearance, in PLOS ONE]

That a curable infection can cause permanent behavior changes is worrisome because this same parasite infects 60 million people in the U.S. alone. Most cases produce no symptoms. But some evidence links chronic infection to schizophrenia and a cluster of other mental illnesses—known unscientifically as “crazy cat lady syndrome."

—Wayt Gibbs

[The above text is a transcript of this podcast.]