To get into a cat and complete its life cycle, the organism changes the behavior of the rat so that it is more likely to be caught and eaten.
Affected rats lose their fear of cats and are even drawn to feline smells, scientists say.
They believe the one-cell parasite, Toxoplasma gondii, could also alter the personalities of humans it infects. About 22 per cent of the UK population carry the organism, which is transmitted through eating uncooked meat or contact with cat faeces. In France, it is carried by up to 87 per cent of residents. Research in Brazil has linked Toxoplasma gondii with hyperactivity in children. The organism's main home is the gut of cats.