Almost every organism on Earth has an associated parasite, and most have quite a few.
We're used to the idea that influenza and malaria microbes can wreak havoc on our bodies, but researchers now realise how they can also infect our minds, intentionally changing the way we think and behave.
Toxoplasma gondii is a tiny microbe that convinces rats to overcome their fear of cats.
This suicidal behaviour will eventually get the rat eaten, passing on the microbe.
T gondii will infect just about any warm blooded animal - including humans.
It causes birth defects in humans, and farmers fear the onset of "abortion storms" where whole flocks of ewes miscarry.
Due to us flushing cat litter down the toilet it's now infecting dolphins too.
T gondii is now the world's most successful parasite, infecting 40% to 60% of humans on the planet.
Scientists in America are now looking at evidence that Toxocara and its relations are responsible for illnesses such as schizophrenia.
This is because excessive production of the neurotransmitter dopamine is associated with schizophrenia - and toxoplasma gondii, uniquely, actually manufactures dopamine.
So how about getting rid of T gondii once it's penetrated your brain? So far, there's no vaccine available except for sheep.
The development of a human vaccine has so far been slow, partly because T gondii doesn't kill.
However, there's exciting evidence that anti-psychosis drugs such as the ones used for mental illnesses such as schizophrenia stop the T gondii from replicating in the brain.
Thankfully, not all parasites are harmful - and a national team of scientists in the U.S.
are investigating the role that other parasites can play in treating Crohn's disease and other chronic autoimmune disorders.
Could infection by parasites be at the root of human behaviour and even mental illness?