Mind how you go (Image: Morgan Schweitzer)

IMAGINE there were a parasite living in your brain – an alien interloper with the power to alter your neurochemistry, manipulate your behaviour and change the way others see you. It might even rob you of your sanity. You are not the only person affected. The creature has taken up residence in the brains of billions of people, and many more are at risk.

This is not fiction. This mind-snatcher actually exists.

We already know that some parasites mess with their host’s mind. The lancet liver fluke, for example, induces suicidal behaviour in any ant it infects, making it climb to the top of a blade of grass and hold on tightly with its jaws until it is eaten by a passing cow. Thus, the fluke gets back inside an animal in which it can reproduce, completing its life cycle. It is not the only parasite capable of such mind control, but generally their targets are insects and other small-brained invertebrates. Influence over a mammal with the brain size of a human was beyond their capabilities – or so we thought. That assumption is being challenged – at least for one parasite.

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You may have heard of it. The microbe in question is Toxoplasma gondii, a single-celled protozoan that infects many birds and mammals but reproduces sexually in just one group: cats. Humans generally acquire it by eating undercooked meat and unwashed fruit and vegetables, or from cleaning litter trays of cats that have recently been infected. Pregnant women and people with immune disorders such as HIV are advised to avoid these risks because …