Imagine a parasite that lodges in the brain and starts to exert a sinister influence over your behaviour. It affects how sexy you feel, or how angry or frightened, even how you dress. Not only does such a creature exist – but it may have infected up to 40 per cent of the population.

Perhaps the most famous example of such “zombie reprogramming” (or neuroparasitology, to give it its proper name) comes from a parasitic wasp that attaches its eggs to the belly of an orb spider. Larvae emerge and release chemicals that zombify the spider, which stops spinning its normal spiral web and instead starts producing a cocoon to hold the baby wasps when they emerge.

Then there is a parasitic fungus called Ophiocordyceps. After being infected with its spores, the Camponotus ant, found in the Brazilian rainforest, develops an unsteady gait, wandering off its normal paths. The creature has become compelled to find a remarkably precise location: a tree about 25cm above the usual ant trails, facing northwest. At noon, it will clamp its jaws on to a leaf in a death grip. Within six hours, it will be dead. A few days later, a tube will sprout from the ant’s head. This is the fruiting body of the fungus that emits the spores, which will infect a new generation of ants.

Worms can do it, too. One species needs to get inside a sheep to reproduce – so it hijacks the brain of another type of ant, which it programmes to climb to the top of a blade of grass every evening and hold on tight. It remains there through the night, waiting for a grazing sheep to eat it. If it’s still there in the morning, it climbs down to avoid being burnt by the sun. But in the evening, the alien instructions take over and it climbs up again.

Scientists are only beginning to understand how such parasites’ controlling abilities evolved – but what is clearer is how they pull off some of the stages. Prof David Hughes, an entomologist at Pennsylvania State University, discovered that one of the chemicals produced by Ophiocordyceps had the power to destroy mitochondria, living creatures’ cellular “power stations”. Once the ants’ jaws lock on to the leaf, they don’t have the energy to unlock them.

Joanne Webster, professor of parasite epidemiology at Imperial College London, explains that many parasites favour the brain, “because it shelters them from the full fury of the immune system”. But, she says, “it also gives them direct access to the machinery to alter the host’s behaviour”.

And those hosts include humans – raising all sorts of tricky questions about whether we are in control of our actions. For example, there is a single-celled parasite called Toxoplasma gondii, which is found in domestic cats, and is estimated to infect 350,000 people a year in Britain. Its effect on humans became the obsession of Jaroslav Flegr, professor of evolutionary biology at Charles University in Prague, who linked it with disturbed behaviours such as reckless driving and a greater risk of suicide.

Unlike the zombified ants and spiders, humans aren’t the intended target of “Toxo”. It can only reproduce in the intestines of cats (new spores are expelled in their faeces). So the parasite’s brain-manipulating powers are focused on getting back inside a cat – and making its host behave in ways that boosts its chances of being eaten.

Rats infected with Toxo, as scientists at Imperial College discovered, actually like the smell of cat urine, instead of being terrified by it. And studies at Stanford University in California have revealed the neural changes that lay behind this transformation. Toxo – which comes in the form of tiny single-celled cysts – was clustered in two areas of the brain: those controlling fear and pleasure. Pathways that normally responded to the smell of cat urine with alarm had been damped down, while the pleasure hormone dopamine, normally released in response to female rodent urine, was now triggered by the whiff of cat. Most recently, researchers have shown that Toxo’s DNA includes two genes that boost dopamine production.

“in my practice, I’ve seen how people have allowed their humanity to drain away. Only it happened slowly instead of all at once. They didn’t seem to mind…All of us – a little bit – we harden our hearts, grow callous. Only when we have to fight to stay human do we realize how precious it is to us, how dear.”

Dr. Miles Bennell”  invasion of the body snatchers.

Human brains have plenty of similarities with those of rats and mice, suggesting that the greater number of car crashes among those with Toxo infection could be due to it damping fear responses. But while female rats show that they find infected males much sexier, the clear effect on humans is to reinforce certain sexual stereotypes. Infected men become introverted, suspicious and more likely to wear rumpled old clothes, but infected woman are just the opposite: in one study, they were usually well dressed when they arrived at the lab for interviews, and also more trusting and sociable.

Do any other microbes provoke similar changes? The leading candidate so far is one of the most common in the world – influenza. Researchers at Binghamton University in New York State, using the ’flu vaccine as a proxy for infection, recorded the behaviour of 36 academic staff two days before, and two days after, getting a jab.

The result was astonishing. Before the vaccination, according to the journal Annals of Epidemiology, they interacted with an average of 54 people a day; afterwards it shot up to 101. Yet the amount of time they actually spent with each person plummeted – from 33 to 2.5 minutes. “Subjects who normally had very limited or simple social lives,” said one researcher, “were suddenly deciding they needed to go out to bars or parties” – the perfect places for a virus to find new hosts.

This may sound terrifying, or at the least unnerving, but there is much that researchers hope to learn from such infections: for example, understanding the way they rewire emotional circuits could provide valuable insights when developing psychiatric drugs. Even zombies, it seems, may have their uses.

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