Some protozoa infect the brain of their host, shaping its behavior in ways most suited to the pathogen, even if it leads to the suicide of the host
The article is talking about a specific type of parasite, Toxoplasma gondii, which reproduces inside cats, but can be carried by any warm-blooded animal (yep, including humans). Rodents are especially good for the protozoa, because they will take over the rodent's brain and cause it to become "suicidal." Rodents are normally frightened of cats, but this protozoa makes it so it likes cats, even actively seeks them out so that the cat can capture the rodent and eat it, allowing the protozoa to reproduce.
So what is its effect on humans? Well, they're not sure. Research has shown that schizophrenic patients are two to three times as likely as other humans to carry this protozoa...however, correlation does not imply causation, and it's impossible to tell if the protozoa causes schizophrenia, or if people with schizophrenia just have a more hospitable environment and are thus more likely to pick it up. In addition, a prospective study tracking the road safety in Czech recruits during their 18 months of compulsory military draft found a rate of accidents six times higher in affected drivers.
The issue of free will is always a sticky one, and things like this make it even more unclear. What if we're subconsciously being manipulated by brain parasites? It sounds stupid, like fiction, but there are protozoa that can manipulate their hosts into doing things they wouldn't normally do. What if I'm not really making my own decisions? It's disturbing, really.
I hope this doesn't get into free will vs. predestination (that wasn't my intent, if it was I would've posted it in the TD forum), and this isn't dealing with predestination anyway, but rather the concept of free will itself. Oh and brain parasites that control your thoughts. That too. That usually isn't theological.