You have ten times as many bacterial cells as human ones in your body, and that leaves out viruses and fungi. Are those non-human creatures in your body part of you?
It’s easy to think of them as ‘not you’, as little Others along for the ride on your body. They are parasites and symbiotes that feed off of us, help us digest food, might even protect us from infections – but they’re not part of me, right?
Enter Toxoplasma gondii, a protozoan parasite that infects all sorts of mammals but really loves getting into cats (the only place it can reproduce). In fact, its talent is finding its way from other mammals into cats. How does it do this?
By altering the behavior of intermediate non-cat hosts. If a mouse is infected, it starts hanging out in open areas. An infected rat actually seeks out cat urine, rather than running from it. Then, presumably, the mice and rats get eaten by hungry cats. In other words, T. gondii changes the behavior of its hosts in order to maximize the chance of finding its way inside a cat.
Of course, T. gondii doesn’t just find itself inside rats and mice on its way to cats. Often it gets into humans, through exposure to pet cats or from eating uncooked meat (a report in the UK discovered that up to 38% of stored meat was infected). Some infected people develop flu-like symptoms, but most people develop no symptoms and the infection remains latent and apparently inactive. For a long time, it was assumed that latent infection in humans had no real effect on the host.