The idea that mind and body are separate pervades our culture and philosophy thanks to Descartes. The actual layout of the human brain seems to agree with this: it is encased in a shell of bone and tough membranes, is visually separated from the rest of the body by the neck, and has a handy defence mechanism called the blood-brain barrier, which (usually) keeps out toxins and potential infections in the bloodstream. So the idea of brain damage or mental illness being caused by infection, in the same way that we might catch a cold, isn’t just strange, it’s downright disconcerting. After all, there’s nothing psychological about sore throats or parasites, right?

Except that there might be. Take parasites, for instance. The single-celled Toxoplasma gondii, which is transmitted by cats, forms tiny cysts in the brain of all its mammalian hosts, including humans, where it then hangs out indefinitely. And Toxoplasma warps the brain, at least in mice; while uninfected mice instinctively fear the scent of cats, Toxoplasma infection makes them slightly attracted to it, and thus more likely to be eaten. This is in the parasite’s best interests, because it can only reproduce in a cat’s digestive tract.

Toxoplasma doesn’t have exactly the same effect on humans – most humans don’t have an instinctive dread of cats – but Toxoplasma infection still correlates with risk-taking and mental disturbance. For instance, the Toxoplasma infection rate is several times higher in people who died in fatal car crashes than in the general population – and dangerous driving is strongly linked with risk-taking behaviour. (Much like mice running towards cats instead of away from them.) Toxoplasma infection is also associated with increased schizophrenia risk; this isn’t true for everyone, though, otherwise cat owners would all start hearing voices. Instead, it’s thought that Toxoplasma acts as a trigger in brains already primed for the disease, for instance in people with a family history of schizophrenia. This does mean we don’t have to worry too much about patting the college cats; just be extra careful on the road.

Even a sore throat may spell trouble for the brain. Infections by Streptococcus bacteria, responsible for ‘strep throat’, can spiral out of control into full-blown rheumatic fever. This affects the joints, heart – and brain. The neurological symptoms are collectively called Sydenham’s chorea, which affects about a quarter of rheumatic fever patients, and range from uncontrollable, jerky limb movements all the way to psychological signs like slowed cognition and altered behaviour. (Historians might know this as St. Vitus’ Dance.) The cause is an autoimmune reaction triggered by the infection: instead of just targeting the invading bacteria, the body’s immune cells mistakenly kill cells in a brain region called the basal ganglia. In fact, autoimmune reactions like this are now linked with many psychiatric disorders in children, especially obsessive-compulsive disorder.

These findings provide graphic examples of why a healthy brain needs a properly functioning immune system, and how a simple bacterial infection can completely derail this. We still won’t catch neural pathology from toilet seats, but the interplay among infections, microbes and the brain clearly shows that looking after the rest of the body is crucial for mental health. Take that, Descartes.