Toxoplasma gondii is a single-celled creature. A protozoa. A parasite. A germ, not entirely unlike malaria. It causes a disease, toxoplasmosis.
However it is a relatively interesting bug. It lives in order of preference in cats, rats and in other mammals.
The cats eat the rats, and the rats eat the cats
Toxoplasma has two usual hosts: Felis domesticus and rodents such as rats. The cat is the preferred host, but about 35% of rats are infected. The cycle is that the toxoplasma bugs are transmitted to the rat through cat urine and faeces that the rodent will encounter in its wanderings, and back to the cat when it catches and eats a rat.
Now germs are neither big nor clever, but being single-celled they do have some advantages: they multiply (by division, heh) rapidly, and they mutate quickly. This allows these bugs to explore a great many paths in evolutionary space. Toxoplasma has hit upon an interesting trick that boosted its chances of propagating. In order to pass from the rat to the cat, the rat has to die. The Toxoplasma somehow gets into the rat's brain, and alters the rat's biochemistry to make it more outgoing, less fearful of novelty. More likely to get eaten.
Living inside me
Germs are simple things, relying on numbers not smarts. Should they end up in any mammal other than their chosen hosts, they will just try to do what their dumb programming tells them to try. There has been speculation that toxoplasma, having shown its versatility in choice of hosts, could have effects on other mammals.
And by "other mammals" we mean us. It has long been known that between 30 and 60 percent of people worldwide carry the bug 1, from contact with dirt or cats (cat poo particularly, but even stroking their fur is not safe).2
Not being the target host, our immune system normally prevents it from doing any harm. It generally only infects HIV+ humans. It is considered an AIDS-defining condition.
It is also dangerous to unborn children ("congenital toxoplasmosis"), causing a variety of nasty defects, and so pregnant women are advised to stay away from dangerous activites such as cleaning cat-boxes for fear that they may catch the bug for the first time, and so would the unborn child, at a time when it would damage the child's development.
When a healthy adult gets toxoplasmosis, they fight it off without much difficulty. After that the bug is often present but latent, in tiny cysts in muscles, nerves and brain, but not having any effect. Or so we thought.
Am I me entirely me?
Here's where it starts to get scary and sci-fi. There is new evidence on the effects of latent toxoplasma on people.
A study in the Czech Republic 3 suggests that people with the latent infection have 2-3 times higher risk of being involved in a car crash.
Toxoplasma carriers have been shown by the same team to have slower reaction times. 4 They are also at greater risk of schizophrenia and manic depression.
If this is true, then toxoplasma is a serious disease, a large public health problem and an economic drain that has gone unnoticed.
Alley cats and sex kittens
A personality survey by the same Czech team shows a correlation between infection and certain personally traits. 5 Toxoplasma seems to change human personality in subtle but statistically noticeable ways.
These changes are different for man and for women.
The tabloid media has summed this up as "alley cats and sex kittens, describing the male changes as "scruffy, promiscuous, aggressive, unattractive to women" and the female as "less trustworthy, more desirable, fun-loving and possibly more promiscuous."
So what does it all mean?
It raises philosophical issues about free will and culpability. What makes me me? Are my decisions mine, or just the result of my biochemistry? How much can my personalty be bent before it's not me doing it anymore? If the bug has shaped who I am, who would I be if it was removed?
Is a particular accidental death in a car crash caused by the presence of the bug in the driver, or by a lapse of human judgement? Or would the driver be a different person without the bug? It doesn't matter where you lay the blame - if Prof. Flegr's numbers and theories are right, then eliminating toxoplasma from humans would save many lives each year, as surely as if the bug had been a straightforward deadly disease like malaria.
It shows that nature is not some monolithic entity that should be anthropomorphised. It is a dense web of independent agents each with their own selfish agenda, which of course conflict. By malice or from pure dumb unintended side-effect, nature is capable of turning up surprises as nasty as any human-made environmental hazard.
I'd be more certain of all this if it wasn't always Professor Flegr's name on the articles. However scientists and journalists seem to be taking him seriously, so it looks like he is a pioneer and not a crank. However this conclusion seems to be becoming stronger not weaker over time.
It should be noted that Toxoplasma is probably less of a problem than it used to be – improved sanitation means that a lower percentage of people come into contact with rats and their diseases than they did five hundred years ago.
1) One source has "The prevalence of toxoplasmosis varies usually between 20-70% depending on the country"
Another lists infection rates as about 80%-90% in France and Germany, and half that in Britain and America. No jokes about national character please!
2) "contact with a cat increased the probability of being Toxoplasma-infected 6.7 times, the eating of raw meat 19.4 times and keeping rabbits 2.3 times"
3) "Increased risk of traffic accidents in subjects with latent toxoplasmosis: a retrospective case-control study."
Jaroslav Flegr et al; 2002.
The study involved 146 "subjects involved in traffic accidents" and 446 of the "general population living in the same area" to come up with an estimate of a 2.65 times higher risk of traffic accidents in the infected.
4) "Decrease of psychomotor performance in subjects with latent 'asymptomatic' toxoplasmosis."
Havlicek J, Gasova ZG, Smith AP, Zvara K, Flegr J. 2001
5) "Changes in the personality profile of young women with latent toxoplasmosis."
Flegr J, Havlicek J. 1999
"Decreased level of psychobiological factor novelty seeking and lower intelligence in men latently infected with the protozoan parasite Toxoplasma gondii"
Flegr J, Preiss M, Klose J, Havlicek J, Vitakova M, Kodym P., 2003.
6) Yes, I did grow up with three cats and a large garden full of dirt. Yes, I am often scruffy. Why do you ask?
Google will give you abstracts for the papers listed
Thanks to eliserh for a clarification.