Around one in four readers of this article has their brain infected with a potentially lethal parasite. The single-celled creature, Toxoplasma gondii, a relative of the agent of malaria, attacks not just humans, but also many other mammals.

There is no cure but for nearly all its victims no symptoms either. Only people already weakened by disease, surgery or age are under threat; and for them the effects may be fatal. Pregnant women, too, are at risk, as their baby’s eyesight or hearing may be affected by the invader. Fortunately, only one in 1,000 infants is born with any signs of damage, although more are at risk of eye infections later in life.

Toxoplasma is a cunning little beast. It enters certain cells in the immune system and directs them to move to the brain. There, it is safe from antibodies and even uses human immune signals to put itself to sleep and out of harm’s way. Like many parasites it needs a third party to complete its life cycle, for our friend the Toxoplasma needs our companion the cat in order to have sex (a tiger or lion also does the job, if one is available).

Once in feline guts the invader produces millions of infective cysts, which can stay alive for months in the excrement (which is why some antenatal clinics screen cat-owners and why pregnant women are advised to avoid cat litter and unwashed vegetables). The creature can also be passed on in poorly cooked meat.

One of Toxoplasma’s talents is subtle indeed. In nature, most cats, big or small, eat only live prey. It would hence be foolish of the parasite to kill its mouse or (for those infections transmitted by lions, its human) host and so destroy its own chance for sex.
Instead, for mice and rats at least, Toxoplasma changes its targets’ behaviour to make them easier prey. A parasitised mouse loses its terror of cats — and, far from running away from the vicious carnivore’s smell as normal, takes an appreciative sniff, turns round and marches proudly towards its feline nemesis.

A grateful parasite then celebrates its nuptials in the satisfied pet’s intestine. The effect is quite specific, for infected mice, fond as they are of cats, still make tracks for safety as soon as they smell a dog. Many parasites manipulate the behaviour of their hosts in this way. A freshwater worm that infests grasshoppers in one part of its life cycle persuades them to find a pond and leap in (grasshoppers cannot swim), while parasitised snails or worms are induced to flaunt themselves to increase their chance of being eaten by the bird or sheep needed to complete their internal enemy’s career.

Malaria-infected mosquitoes, like rabid dogs, bite more than before. To match that, a recent study in Kenya showed that children with malaria are twice as attractive to mosquitoes as are those free of the illness. A series of molecular signals in a host’s brain that swing into action when its foe arrives have been implicated in such shifts.
Claims that Toxoplasma alters our own mental chemistry are shaky at best. Some say that people who carry the little pest become more impulsive and have slower reaction times than before (which is bad news when there are lions about) and that they are as a result twice as likely to have car accidents (vehicle fatalities are, by the way, twice as frequent in France as in Britain).

It is possible that changes in the levels of the nerve transmitter dopamine are to blame. As shifts in that chemical are also associated with certain forms of schizophrenia, a few psychiatrists suggest that infected people are more at risk of the disease. There are even some hints at a tie between schizophrenia and cat ownership.

Whatever the truth of those wider assertions, the sneaky little creature’s effect on the relationship between cats and mice is a perfect and objective instance of Richard Dawkins’ metaphor of religion (and fanaticism in general) as a mental virus whose only instruction is “copy me, however much it costs”. For mice, the price is high for, like holy warriors, they lay down their lives for an irrational belief caused by a brain infection.
Many important scientific questions arise from this work: when lambs lie down with lions, is the parasite to blame? And are England fans and jihadis more likely to be cat owners than those of us bored by football or faith?

The Daily Telegraph