Narration So who's in charge here ... is it the cat ... is it the rat ... or is it something else entirely?

Dr Jonica Newby As a vet, I've been fascinated by the discovery that there's a tiny parasite capable of taking over this rat's brain.

And It makes you wonder could they also be influencing the behaviour of humans? Are we all subject to alien mind control?

Narration And here's evidence - alien bugs ready to be implanted in a brain.

Dr Jonica Newby So what are these little wrigglies.

Dr Nicky Boulter These are the Toxoplasma tachyzooites which is the infective stage of the parasite, and they can travel throughout your body predominately to your brain where they can form cysts and that's where they live for the rest of your life.

Dr Jonica Newby The rest of your life?

So you're saying these could be in my brain?

Dr Nicky Boulter Well, quite possibly if you've ever had cats or eaten undercooked meat. 30 to 40% of the population is infected with this parasite.

Narration We've known about the strange life cycle of Toxoplasma for some time.

In its juvenile stage it invades and develops inside the muscle or brain of any mammal or bird ... to reach its adult stage, it then needs to be eaten ... by a cat.

But what nobody realised is that the little parasite was playing an active part ...

... until in 2000, scientists in England stumbled on something amazing.

Dr Jonica Newby Well, to demonstrate what was discovered, I've organised the assistance of a stunt rat ... and a stunt cat. Now, this is actually my cat and he has no teeth and arthritis so I think he's pretty safe - but just in case, I'm going to put him in this cage.

Narration Normally rats are innately terrified of cats.

But when rats were infected with Toxoplasma, amazingly, they seemed to lose all fear of cats ... in fact, they seemed perversely attracted to them.

They'd been turned into suicide rats.

Dr Nicky Boulter Suicide rats, mind control rats. Basically it's a bit like the X files you know that these things can live in your brain and dictate how you behave.

Narration So had they lost all fear, or only the fear of their mortal enemy? What was going on?

Earlier this year, in sunny California, scientists in Stanford went looking for the answer.

In this experiment, a small piece of cat collar is placed in a clip. This is a normal rat - look how he reacts to cat odour.

But this is how infected rats behave.

When the scientists then looked inside the rats brain, it turned out toxoplasma had preferentially set up home in the amygdala - the brains main fear centre.

More incredibly, the rats didn't lose their fear of other things like electric shocks ... nor their sense of smell.

Dr Nicky Boulter It blows me away - to be so precise in targeting the area in the brain that will make a rat less fearful of a cat. It's just phenomenal. I mean, how clever is that to not affect the other behaviours that are going to affect the rodents general well being because they don't want the host to die, unless its eaten by a cat.

Narration It begs the question - can they do the same in humans?

Nearly half of us are hosting these things, and we've just found out, they're not as quiet in there as we thought.

Dr Nicky Boulter Every now and again its believed that parasites actually do liberate from the cysts and wriggle around in your brain for a little while before they go back into cysts.

Dr Jonica Newby It's a bit freaky, isn't it?

Dr Nicky Boulter It's very freaky.

Narration One research team in Europe has found Toxoplasma infected women are more likely to be promiscuous.

Dr Nicky Boulter There have been studies as well that show you're more likely to be in a car accident, either as a pedestrian or a driver of a vehicle. Two and a half times more likely to be involved if you've got the parasite.

Dr Jonica Newby Well, the findings come from just one research group, and the rule of science says results need to be replicated, so we'll remain open minded about the effects of Toxoplasma on humans for now.

Narration So the moral is; read up on hygiene and make sure you follow all the recommendations for avoiding parasitic infections.

Dr Jonica Newby Though let's face it. If you really want to know about parasites that can take over human behaviour, you don't have to look much further than this. Do you, you little parasite.