You Might Be Hosting a Parasite Right Now. Here’s How to Tell

There are more parasites than any other organism on Earth—which is one of the reasons Emmitt Jolly finds the common but misunderstood creatures so fascinating. “That is the most successful form of life,” says Jolly, a professor of biology at Case Western University. People often assume that honor would go to insects, but “the reality is that every insect you find is parasitized by something.”

Sometimes, humans are too. Nearly 850 species of parasites are known to infect people; some, like the potentially deadly plasmodium parasite that causes malaria, are well-known. Others fly more under the radar.

We asked Jolly and other experts how parasites are transmitted, what it feels like to be infected, and what you can do to protect yourself.

How people get parasites

Parasites truly live up to their name. “They take something from the host but do not give anything back,” says Bobbi Pritt, a professor of laboratory medicine and pathology and the chair of the division of clinical microbiology at Mayo Clinic. While some cause serious, potentially life-threatening illness, others aren’t harmful, and people don’t always realize they’re infected. Take the flat, ribbon-like tapeworm, which sneakily absorbs your nutrients: “You may not even know you have a tapeworm until part of it comes out [in your stool],” Pritt says. “Of course, that’s usually very traumatic and horrifying, but up until that point, it’s just a passenger within you, and you might not know that it’s there.”

Parasites are most common in tropical and subtropical parts of the world, like sub-Saharan Africa, Asia, and the Caribbean. They thrive in areas with poor sanitation and hygiene, and people can get infected in a variety of ways, Pritt says, including through insect bites, by drinking unclean water or eating undercooked meat or vegetables, via sexual contact, and by walking barefoot on contaminated sand or soil.

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Parasites are less common in the U.S. than in developing countries. “That’s because of interventions over the years like safe water and urbanization and clean toilets, and the fact that we don’t necessarily walk barefoot in areas where there could be snails and worms,” says Dr. Alfredo Mena Lora, chair of the department of medicine at Saint Anthony Hospital in Chicago and an infectious disease expert. “Climate, health—all those things help reduce parasitic infections in the U.S.” Yet there are still millions of new infections in the U.S. per year. 

Many of these infections are easily treatable with antiparasitic medications. Some, however, have such subtle symptoms that they often go undiagnosed, Lora says. 

A closer look at parasites

Here’s what to know about a few of the parasites experts see pop up in humans.

Toxoplasma

If you have a cat, you’ve probably heard that women who are pregnant shouldn’t clean litter boxes. That’s because of the risk of toxoplasmosis, an infection caused by the parasite Toxoplasma gondii, which is transmitted through cats’ feces. “This is probably one of the most proliferative parasites in terms of their ability to infect a lot of different things,” Jolly says. Most parasites only affect a small set of species, but Toxaplasma gondii can strike cats, humans, birds, livestock, rodents, and other mammals.

Research suggests that 30% to 50% of people around the world have been infected with this parasite. Most people don’t experience any notable symptoms, but for those with weakened immune systems as well as women who are pregnant, it’s a different story: The infection can cause the fetus to have complications like blindness, epilepsy, anemia, and damaged brain tissue.

Plus, a few small studies suggest toxoplasmosis can affect human personality, making people more prone to intermittent explosive disorder and increased aggression, more likely to have car accidents, and more sexually promiscuous.

Tapeworms

Tapeworm infections have declined significantly in the U.S., but they still occur, mainly when people eat raw or undercooked meat that contains Taenia saginata (beef tapeworm) or Taenia solium (pork tapeworm). Once the larvae enter your intestines, they turn into big worms with lots of little segments that expand and grow in your stomach. “When you eat,” Jolly says, “they eat.”

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People often don’t realize they have beef tapeworm disease, though they might have some mild abdominal symptoms or lose weight. But pork tapeworm can lead to cysticercosis, which happens when larvae migrate outside of the intestines, affecting the brain and eyes. That can trigger seizures, headaches, confusion, and other neurological issues, which is typically treated with either antiparasitic medications or even surgery for tapeworm-related cysts in the eyes.

Giardia

Years ago, before he became the parasite expert he is today, Jolly almost made an amateur mistake: He recalls leaning forward to drink some of the crystal-clear mountain water raining down in front of him from a waterfall at Yosemite National Park. “I was thinking, ‘Look how pretty—this is probably the perfect drinking water,’” he says. “My buddy from California hit me and said, ‘Man, there’s giardia in this.’ I’m very happy I did not drink the water—let me just say that.”

Jolly’s hike almost certainly would have taken an unpleasant turn. Giardia—often called “beaver fever” because it can spread through the rodents’ feces—causes severe diarrhea, stomach cramps, nausea and vomiting, bloating, dehydration, and irritability. When the parasites invade the intestines, they can proliferate rapidly, he adds, covering the entire lining of your stomach wall.

Hookworms

Hookworms are ancient parasites that still pop up in the U.S., especially in the South. People are often infected when they walk barefoot and step on animal or human feces full of larvae that then burrow into their skin. (Yet another reason to always wear shoes.) “What happens is the hookworm is waiting for a host in the sand,” Jolly says. “Once they’re exposed to skin, they pass through directly into the circulatory system, until they find the way to the intestines, where they enter and put their heads in and produce offspring.” 

That doesn’t mean, however, that you end up with more hookworms: Worms’ eggs typically pass out through your feces. “If you have seven hookworms, you have seven hookworms,” Jolly says. “You’re not going to end up with 17,000, unlike many of these single-cell organisms, like giardia.” Unless you have a whole lot of hookworms—which live for six to 10 years—there’s a good chance you won’t ever even know you’re infected, since light infections are asymptomatic. In more severe cases, the worms can cause stomach issues, anemia, and fatigue.

Pinworms

These white, parasitic worms are straight out of a nightmare: They come to life while the person they’ve infected is asleep, emerging from the intestinal tract to lay eggs on the skin around the anus before crawling back in. “The end result is that people begin to itch and scratch,” Jolly says. “You’ll see these little kids, and they’ll just be scraping their butts on the floor”—a telltale sign of infection.

Pinworms are highly contagious, and often, when one person in the family has the parasite, it usually becomes “a family affair,” Jolly says. As you might imagine, those who are infected often experience psychological distress. “Here you are every night, and these things are crawling in and out of your anus,” he says. “You’re concerned and you’re tired, you can’t go to sleep, you start waking up—people become anxious, very much so.”

While antiparasitic medications can kill pinworms, it’s also important to thoroughly clean all linens, such as bedsheets and towels, if you get infected, since eggs can survive on surfaces for up to three weeks.

How to protect yourself from parasites

You can take certain steps to lower your chances of becoming an unwitting host to one of these nasties. Practicing basic hygiene is key, Lora says. Wash your hands with soap regularly, use insect repellent, always wear shoes outside, and make sure your food is cooked thoroughly.