More on the parasite Toxoplasma gondii - the one that breeds inside your brain cells and turns you into a car-crashing schizophrenic. It infects humans, but the primary host is the cat. Animals are infected by eating infected meat, by ingestion of faeces of an infected cat, or by transmission from mother to fetus.
Up to one third of the world's population is thought to carry the infection. Including Martina Navrátilová, for one.
At first, the infection usually causes a mild flu-like illness, or nothing at all. After the first few weeks, the parasite rarely causes any obvious symptoms in otherwise healthy adults. However, people with a weakened immune system, such as those infected with HIV, and fetuses, may become seriously ill, and it can occasionally be fatal. The parasite can cause encephalitis (inflammation of the brain) and neurologic diseases and can affect the heart, liver, and eyes. ( For some reason, Toxo-infection also means you're almost 3 times more likely to have a boy-child, instead of girls )
But the only reason it kills you is because the mechanism it's using to stop you dying, has been been thrown completely off the rails.
Toxoplasma doesn't want you to die. Thus, it uses your own immune system to keep its own numbers in check - deliberately ensuring that any excess Toxoplasma get destroyed by your immune system before they can invade and damage other brain cells.
It does this by subtly tipping the desperate immune response towards white blood cells that eat Toxoplasma as they wander between brain cells. That stops the infection getting any worse, because the ones that are already sitting in little groups of 8 inside your neurons are smugly immune to such attacks, and are willing to wait it out. That way you don't die of out-of-control brain damage before you get eaten by a cat (yes, it isn't supposed to be infecting *you* - it's supposed to cycle between cats and rodents). The symptoms include increased risk-taking - in rats, that means going out to investigate this interesting 'cat' smell, whilst the cat is still around. In humans, you get traffic conditions in Rome.
Unfortunately, if you're a fetus or immune-compromised, you don't have any of those particular white blood cells and the result is massive brain destruction, as the Toxoplasma breed out of control, infecting and destroying every brain cell they come across. That's what happened to the guy in Trainspotting, and one reason AIDS was closely associated with brain damage before the Toxoplasma link was understood.