Here's how the bug works: Humans ingest Toxoplasma gondii , a common relative of malaria, which, at first, only makes itself known through mild flu symptoms. These symptoms have been known to linger in individuals with compromised immune systems, such as those affected by AIDS, but, in the majority of cases, the symptoms pass, the host feels better, and the bug is forgotten.

Have you ever felt guilty and not understood why? Or felt that you're easily distracted? Most have. A study by the U. S. Geological Survey shows that a parasite that invades human brains and cat intestines could be to blame.

Dormant in the host's brain, it forms cysts. Research suggests that the parasite begins to affect how the brain responds to dopamine, a hormone that works as a neurotransmitter. And, gradually, it changes the host's personality.

Scientists have recorded four ways in which T. gondii affects the host. First, individuals with the parasite are more likely to experience guilty feelings, classified in studies as a type of neuroticism, than those who are not infected.

Second, researchers have found that the parasite's human hosts are more likely to be uncoordinated. One study demonstrated that both drivers and pedestrians who have been in traffic accidents are three times more likely to have been exposed to T. gondii.

Third, those who are infected with T. gondii are less likely to seek out new experiences. In researchers' words, they are less prone to "novelty seeking" and are more likely to demonstrate characteristics of "uncertainty avoidance."

And fourth, and perhaps most frighteningly, the presence of Toxoplasma gondii in human hosts shows a correlation with schizophrenia. People who suffer from schizophrenia are, in fact, three times more likely to carry T. gondii than those who do not. While unsettling, the correlation is not altogether surprising; schizophrenia is related to abnormalities in the way the brain receives dopamine, the hormone that Toxoplasma gondii also affects.

Scientists have observed other trends as well, including skewing sex ratios so that in heavily-affected populations, more males are born.

So what is Toxoplasma gondii, and what does it want with us? Most often described as a "cat parasite," the bug has three hosts: humans, cats, and rodents.

In cats, the parasite lives in the wall of the small intestine and is spread through the cats' feces. Then other animals ingest it, including rodents and humans; in both, it forms cysts in the brain, liver and muscle tissues.

The brain cysts affect the rodents even more dramatically than they affect humans. Research conducted by Joanne Webster at Imperial College, London seems to indicate that T. gondii causes rodents to draw predators' attention by wandering in the open; in some cases, it seems that they may even seek out the smell of cats. Of course, if a cat eats the rodent, Toxoplasma gondii's lifecycle starts over within the cat's intestines, so it's in the bug's best interest to hijack these rodents.

Interestingly, the dopamine-blocking Haloperidol, the drug used to treat schizophrenia in humans, reverses the fatal effects of T. gondii in rodents.

Over the last five years or so, evidence has been building that some human cultural shifts might be influenced, or even caused, by the spread of Toxoplasma gondii. Kevin Lafferty, the author of the USGS study, said in the September 2006 issue of soundwaves, the USGS newsletter, "The geographic variation in the latent prevalence of Toxoplasma gondii may explain a substantial proportion of human population differences we see in cultural aspects that relate to ego, money, material possessions, work, and rules."

In other words, the values and practices of different societies around the world might be shaped by the prevalence of Toxoplasma gondii among the members of that society. So if a large percentage of a particular population has the parasite, then that population is more likely to exhibit certain behavioral characteristics ranging in seriousness from unfounded feelings of guilt to schizophrenia.

This is certainly a frightening observation, given that, in same geographic areas, nearly 60 percent of the population is infected!

In some countries, pregnant women are tested for the bug before giving birth. In the United States, 12.3 percent of women tested carried the parasite, and in the United Kingdom only 6.6 percent were infected. But in some countries, statistics were much higher. 45 percent of those tested in France were infected, and in Yugoslavia 66.8 percent were infected!

It's not yet known whether Haloperidol, the drug used to suppress schizophrenia and block T. gondii's effects on rodents, also works to reverse the parasite's less-serious effects in humans. In fact, not a lot is clear at this point. As most of the scientists looking into T. gondii have pointed out, a lot more research will have to be conducted before we fully understand how much the bug has influenced human culture.

So for now, we'll just have to wonder: how much has the bug affected our lives today?

John Light is a contributor to several Ohio news and arts publications. He is currently enrolled as a senior at Oberlin College, where he works as Editor-in-Chief of the Oberlin Review, an independent, student-run web and print news source.