 Of all foodborne diseases, a brain parasite called toxoplasma is ranked as the fourth leading cause of hospitalization and the second leading cause of death in the United States. Nearly a quarter of us have already been infected. One in three of us, by the time we hit our 50s, although we may then be stuck with this thing in our brain for life, our immune system is so good it can't get in a bay for most healthy people it's never able to rear its ugly head, or at least overtly. Studies have now revealed associations between toxoplasma infection and the presence of various psychiatric disorders in humans, schizophrenia, bipolar, suicide, self-harm, and memory impairment when we get older. How can a tiny parasite alter our very behavior? I talked about how the rabies virus hangs out in the saliva while specifically targeting the emotion center of the brain to drive animals into a fury so they'll effectively do the viruses bitting to bite others to transmit the virus, or the famous zombie ant brain fungus that takes over the animal completely. These are examples of so-called parasitic manipulation, where the parasite manipulates the host to enhance its own transmission by altering host behavior. And toxoplasma is perhaps one of the most convincing examples of a manipulative parasite of higher animals like us. Once the parasite thrives in cats, chronically infected rodents no longer respond to cat odor with fear. And indeed, the physical response is reversed to attraction. Mice become attracted to the smell of cats, serving the parasite up on a silver platter. The parasite manipulates the rodents brain to turn their innate version to cats into a suicidal fatal feline attraction. Mice become attracted to cat pee, and such fatal feline attraction appears specific towards cats that don't become attracted to pee in general, remain indifferent to rabbit pee, and continue to be turned off by other predator pee. So on one hand, the parasite manipulation appears incredibly specific, but the parasite doesn't just want the mouse to seek out the cat, but get eaten as well. And so there are these general effects to impaired motor function, slower response time, memory and coordination. So when the cat pounces, the parasite tries to make sure the mouse doesn't get away. It's like when California sea otters get toxoplasma, they're more likely to get eaten by a shark. It's not that the parasite wants to get into the shark, it may just be a byproduct of the kind of general cognitive deficits that's so helpful for the parasite in other contexts. It's like when humans get toxoplasma, we start liking the smell of cat pee more, too. Isn't that wacky? The parasite knows just what strings to pull, but it's the more general effects we're concerned about. We don't need to worry our newfound appreciation for saber-to-tiger urine is going to get us eaten, but mucking with our reaction time, that could be a problem. That could be why multiple studies have shown more traffic and worksite accidents among those who are clinically infected. But it may not just be our slowed reaction time. The parasite appears to also affect subtle behavioral alterations like personality alterations that make us more likely to take risks. Great for the parasite in the cat and mouse game, but not so much if we're driving a car, or wondering whether or not to take the next drink. Maybe one reason people with this brain parasite get into so many car accidents is that it may make people engage in riskier behaviors like excessive alcohol consumption. We typically think of malaria as being humanity's greatest killer parasite. However, when we take into account the hundreds of thousands of deaths that occur due to the increased probability of traffic accidents, working accidents, suicides, and possibly other side effects to the infection, maybe this supposed asymptomatic latent toxoplasma infection that is infested one in four Americans could easily take malaria down a notch. Before I get into how to prevent and treat the darn thing, what might these other side effects be? How exactly does toxoplasma manipulate behavior? Well, one clue we got decades ago is the rise in dopamine levels in the brain. You can show it right in the Petri dish of infected brain tissue. Turns out that these parasites actually have an enzyme to make dopamine from scratch, which they then release into the surrounding brain tissue. Why do we care? Because elevated dopamine is a characteristic of schizophrenia. That's how nearly all modern anti-psychotic drugs work by trying to bring dopamine levels back down, either inhibiting dopamine receptors or decreasing the level of dopamine in our brain. So is it possible that the increased dopamine accumulation and release observed during toxoplasma infection might increase the risk of schizophrenia? Well, that should be easy to figure out. I mean, do schizophrenics have an increased prevalence of infection? The increased prevalence of toxoplasmosis in schizophrenics has been demonstrated by at least 50 studies.