When a mouse smells the heady aroma of cat urine, it will typically run in the sensible direction: away. But if that mouse is infected with a parasite called Toxoplasma gondii (or Toxo for short), it will instead make a fatal beeline towards the source of the smell, and the jaws of the feline that created it.

Its suicidal behaviour is Toxo’s doing. This parasite infects a wide variety of mammals but it can only complete its life cycle in the guts of a cat. If it ends up in a mouse or rat, it’s stuck—but not for long. It permanently hacks the rodent’s brain chemistry, so that it reacts to the smell of cat with something more like