Everybody loves cats. For centuries, man’s love affair with these furry felines has gone beyond their utility as a means of eliminating vermin. In our modern age, the most popular videos that people waste countless hours on are of cats doing silly things. The most beloved memes often involve cats, including one that shows an angry cat in front of a salad. Undoubtedly, cats are beautiful, furry, and affectionate creatures with which most people are more than happy to share their homes. However, unlike dogs, which are loyal and love their owners to a fault, cats can be fickle, temperamental, and act like they own the owner and not the other way around. Despite this, people cling to their cats fiercely, despite all the bite marks and scratches, not to mention the insufferable yowling at night. Is it really their personalities we are in love with, or is there something else going on?

As it turns out, your cat may be controlling your mind through a sophisticated form of biological warfare. It does this by infecting you with a microscopic brain parasite called Toxoplasma gondii. Toxoplasma, or toxo for short, is a protozoan (one-celled organism) that reproduces in the gastrointestinal tract of cats and can infect many animal hosts, including humans. Only cats and related felines are toxo’s definitive hosts where it can reproduce and form oocysts, the infectious form of the parasite. Almost all other mammals are just intermediate hosts. Oocysts are subsequently shed in the cat’s stool. When a hapless owner cleans the cat’s litterbox or said cat rubs its behind on its owner’s face, some of these oocysts are ingested or inhaled and infect the human host. These oocysts germinate in the gut of the intermediate host and invade tissues, including the muscles and the brain, where they form cysts. Humans can also be infected with toxo by eating undercooked or raw meat from other animals containing toxo cysts. Either way, once the toxo gets into your brain, that’s when the possible mind control happens.

Scientists first realized that toxo may be controlling its hosts by studying its effect on mice. Researchers noted that mice infected with toxo in their brains lost their fear of cats. In fact, these mice were attracted to cat scents and even the smell of cat poop. Toxoplasma protozoans secrete certain types of neurologically active chemicals that cause changes in the host’s brain, affecting its behavior. It is thought that the parasite does this to mice in order to make it easier for cats to eat the mice and get infected. The toxo literally turns the mice into its zombies in order to facilitate its own reproduction, a phenomenon appropriately termed the Fatal Attraction Syndrome.

What about humans? Up to one-third of the human population is infected by toxo, as evidenced by the presence of antibodies in our blood. Aside from mind control, toxo is very good at fooling the body’s immune system. When our immune cells recognize toxo in our blood and tissues, they swallow it whole and try to digest it. The toxo hijacks the cell’s life processes to prevent it from getting digested. Toxo then turns the cell that swallowed it into its new house, where it can make more of itself. Due to its ability to evade the immune system, most people infected with toxo don’t feel any symptoms and will never know it’s there. Some people do develop a fever, headache, and muscle pains when they first get infected, but these symptoms are generally self-limited and go away without any treatment. Eventually, the immune system learns to tolerate the toxo. It lives in most people without causing any further problems.

There are two types of patients for whom toxoplasma can have devastating health consequences: Pregnant women and people with damaged immune systems. Pregnant women who get toxo for the first time can pass the infection to their unborn baby, where the toxo infection can cause birth defects, including damage to the brain, kidneys, blood, liver, and other organs. This can cause the mother to miscarry or deliver prematurely. People with damaged immune systems, such as those with advanced, untreated HIV, can end up with a fulminant form of toxo that can cause deadly seizures along with blindness. These patients will need treatment with antibiotics until their immune systems recover enough to control the parasite.

So, is toxo really controlling your mind just like what it does with mice? Scientists aren’t quite sure, but some systematic studies do point toward some sort of effect on our behavior. Clearly, your cat isn’t going to eat you, and so the toxo is stuck in your brain with nothing else to do.

One study showed that people infected with toxo tended to have slightly higher IQs than those who didn’t test positive for toxo, but only for people with certain genetic traits. It is intriguing that toxo might make you smarter, but only if you have the correct genetic makeup. Another study showed that toxo had different effects on the sexes, although these differences were small. Women infected with toxo tend to have higher IQs, are more social, affectionate, and law-abiding, while infected men tend to have lower IQs and are characterized as more conservative, loyal, and mild-tempered. Both men and women infected with toxo tended to be more neurotic. Neurotic people are more likely to be anxious, fearful, temperamental, and have more negative emotions. How toxo does this to people in terms of a biological mechanism remains unclear, but it probably has to do with its manipulation of brain chemicals such as dopamine and serotonin. What does make sense is that if you are neurotic, you’ll probably want to keep your cat, a.k.a. the toxoplasma factory, around for a very long time for constant “therapy sessions.” One other fascinating observation is that countries with high rates of toxoplasma infection have parallel high rates of neuroticism. This suggests that toxo is not only affecting individual human behavior, but it could be influencing the evolution of human culture. Not bad for a one-celled organism and its feline host.