In central Europe, a speeding car ploughs into the central reservation, killing the driver. At exactly the same moment in rural Ireland, a shepherd watches in despair as two thirds of his ewes miscarry. Meanwhile, on a garbage-strewn street in east London, a cat pounces on an unwary rat.
Three species, in three locations, dead in three different ways. But all victims of the same killer.
The idea of a sinister force able to bend people to its will has long been a staple of science fiction. Invasion of the Body Snatchers, Shivers, The Manchurian Candidate: all drew their horror from the alarming thought that other humans might have been subsumed by some nefarious agent.
We fear such people, we say they lack humanity. They are automaton, puppet, zombie. Beware, then: the reality is that such mind control not only exists but is widespread – certainly in other species, and perhaps in our own too.
Toxoplasma gondii is a microorganism that likes nothing better than to set up residence inside a warm-blooded host, typically a rat. The only time it gets particularly fussy over its surroundings is when it comes to sex, which can only take place in a cat. That poses a bit of a problem for the parasite, as rats aren't known for their fondness of the feline race.
But T. gondii has a very clever trick up its sleeve: it rewires the rat brain. Rodents infected with T. gondii lose their instinctual fear of cats and engage in reckless risk-taking that sooner or later puts them into the jaws of a passing cat.
Should we be concerned that around 40% of the human race is infected with T. gondii?
For a long time, nobody thought so. T. gondii is known to cause birth defects and precipitate spontaneous abortions, and for that reason pregnant women are warned to stay away from cats. But it's also one of any number of bugs that we pick up in our lifetime without experiencing any noticeable effects.
That perception changed when a Czech parasitologist named Jaroslav Flegr decided to look for evidence that T. gondii's mind-meddling extends beyond rats. Testing the blood of drivers responsible for causing traffic accidents, he discovered they were two and a half times more likely to have been exposed to T. gondii than the general population. Might these drivers have been unwittingly egged on by a tiny parasite?
The evidence is that T. gondii can't exert the same level of control over humans that it has over rats. It has the right tools, but it doesn't quite know how this particular model works.
Nevertheless, it's an unsettling thought. While we're reconciled to the idea of disease in our body, diseases of the mind still carry a tremendous stigma. Maybe this is because we've grown to view the body as a fleshy extension of our mind. Our identity – the human soul – is incorporeal, and immutable. Cut off a toe and you're still you. The toe isn't.
We project this fantasy even beyond death, insisting that as our mortal remains crumble into compost, the fragment that is "us" somehow lives on.
We've just about reached the point where we can start talking openly about mental illness, that there is some ideal state of mind from which we can get displaced, that the gears of the mind sometimes run too fast or too slow. But we still prefer that ideal state.
So ask yourself this: if I could test your blood, and reveal that, most likely, a parasite was responsible for your convivial nature, or your love of roller-coasters, would you want it removed?
Our understanding of behaviour-modifying parasites is in its infancy. We're still coming to terms with what effects they have, let alone how they do it. Future research will focus on parasites suspected of causing mental disorders – T. gondii has already been implicated in the development of schizophrenia.
In many ways, this echoes work done over a century ago as scientists got to grips with how invisible microbial pathogens cause disease. Eventually we not only came to terms with these bugs, but embraced them. Jaunty adverts on TV remind us to top up our "friendly bacteria".
In the future, will T. gondii's descendents be commodified to offer us bacterial pick-me-ups? Now that we're used to viewing our bodies as an ecosystem, can we make space in our minds for another voice?
AMC's acclaimed drama The Walking Dead aired on terrestrial TV on Sunday, the story of a group of survivors struggling to keep their humanity in the face of a zombie apocalypse. In the run up, Channel 5 took out full-page ads in newspapers to ask readers "WHAT MAKES US HUMAN?".
With the triumph of microbes such as T. gondii, the distinction may not be as clear as we'd like it to be.
http://www.guardian.co.uk/science/blog/2011/apr/12/zoology-infectiousdiseases
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