Parasites get into your body through four main routes: swallowing contaminated food or water, skin contact with contaminated soil, bites from infected insects, and close contact with infected animals or their waste. Some of these routes are more common than you’d expect, even in the United States, where nearly 14% of the population shows evidence of exposure to just one type of parasite (Toxocara, spread by dogs and cats).
Contaminated Food
Eating raw or undercooked meat is one of the most direct ways to pick up a parasite. The roundworm Trichinella, for example, lives in the muscle tissue of animals like pigs, bears, wild boar, and walrus. When you eat that meat without cooking it thoroughly, the larvae survive and establish themselves in your body. Symptoms can start within one to two days or take up to eight weeks to appear, depending on how many larvae you consumed.
A food thermometer is the most reliable tool for prevention. Freezing pork at -15°C (5°F) for 20 days can kill the worms, but this doesn’t work for wild game because some species of Trichinella are freeze-resistant. Curing, salting, drying, smoking, and microwaving also fail to kill the larvae consistently. Homemade jerky and sausage made from wild game are particularly risky if the meat wasn’t cooked to a safe internal temperature first.
Tapeworms follow a similar path, entering through undercooked beef or pork. Raw or lightly prepared freshwater fish can carry liver flukes and fish tapeworms. Even produce can be a source: fruits grown near livestock pastures can become contaminated with parasites shed in animal feces. If that fruit isn’t washed properly before eating, you’re swallowing the parasite along with it.
Contaminated Water
Cryptosporidium is the leading cause of waterborne disease in the United States. This microscopic parasite lives in water, food, soil, and on surfaces that have been contaminated with infected feces. What makes it especially stubborn is its ability to survive in environments that would kill bacteria and viruses, including chlorinated swimming pools and treated tap water in some circumstances. Swallowing even a small amount of contaminated recreational water (pools, lakes, rivers) is enough.
Giardia follows a similar fecal-oral route. Both parasites produce tough outer shells that protect them in the environment for long periods. Symptoms from Cryptosporidium typically show up about a week after exposure, though the range stretches from 2 to 28 days. Giardia takes a bit longer, usually one to four weeks before you notice anything.
Untreated well water, streams, and lakes in both rural and urban areas can harbor these organisms. Travelers to regions with less reliable water treatment face higher risk, but outbreaks happen regularly in the U.S. as well.
Walking Barefoot on Contaminated Soil
Some parasites don’t wait for you to swallow them. Hookworm larvae hatch in warm, moist soil and develop into a form that can actively bore through your skin. The most common way people get infected is simply walking barefoot on contaminated ground. The larvae penetrate the soles of your feet, enter the bloodstream, travel to the lungs, get coughed up and swallowed, and eventually settle in the small intestine.
Soil becomes contaminated when infected people or animals defecate outdoors, which is why hookworm remains a major problem in tropical and subtropical regions with inadequate sanitation. According to the World Health Organization, soil-transmitted worm infections are among the most common infections worldwide. Wearing shoes outdoors in areas where sanitation is poor is one of the simplest and most effective preventive measures.
Insect Bites
Bloodsucking insects act as delivery vehicles for parasites that live in human blood and tissue. The insect picks up the parasite during a blood meal from an infected person or animal. The parasite then multiplies inside the insect and gets injected into the next person it bites.
Mosquitoes are the most significant vectors. Anopheles mosquitoes transmit malaria, one of the deadliest parasitic diseases globally. Several mosquito species also spread lymphatic filariasis, a parasitic infection that can cause severe swelling of the limbs. Triatomine bugs, sometimes called “kissing bugs” because they tend to bite near the mouth at night, transmit the parasite responsible for Chagas disease. An estimated 300,000 people living in the United States carry the Chagas parasite, most of whom were infected in Latin America before immigrating.
Unlike food or waterborne parasites, you can’t prevent vector-borne infections through cooking or hand hygiene. Bed nets, insect repellent, and protective clothing are the primary defenses in areas where these insects are active.
Pets and Animals
Dogs and cats are common sources of parasitic exposure, particularly for children. Young puppies and kittens are more likely to carry roundworms and hookworms, shedding parasite eggs in their feces. Those eggs can survive in soil, sandboxes, and yards for months. Children who play in contaminated areas and then touch their mouths are especially vulnerable.
Toxocara, a roundworm carried by dogs and cats, is remarkably widespread. CDC data shows that almost 14% of the U.S. population has been exposed to it. In most cases, the infection causes no symptoms or only mild ones. But in roughly 70 people per year, most of them children, the larvae migrate to the eyes and cause vision loss or blindness.
Cats also carry Toxoplasma gondii, one of the most common foodborne parasites in the United States. You can pick it up by handling cat litter without washing your hands afterward, or by gardening in soil where cats have defecated. Toxoplasma symptoms typically appear 6 to 10 days after exposure, though many healthy people never notice they’re infected.
The simplest protection: pick up pet waste promptly, wash your hands after handling animals or cleaning litter boxes, keep pets on a regular deworming schedule through a veterinarian, and supervise young children playing outdoors in areas frequented by animals.
Person-to-Person Spread
Intestinal parasites that follow a fecal-oral route can spread directly between people. This happens when an infected person doesn’t wash their hands thoroughly after using the bathroom and then prepares food, touches shared surfaces, or has close physical contact with others. Daycare centers, nursing homes, and other group living environments see higher rates of parasites like Giardia and Cryptosporidium for exactly this reason.
Food service workers who practice poor hygiene or work in unsanitary conditions can also pass parasites to large numbers of people through the food they prepare. A single contaminated worker can trigger an outbreak affecting dozens or hundreds of diners.
How Quickly Symptoms Appear
One of the tricky things about parasitic infections is the wide range of incubation periods. You might not connect your symptoms to the exposure that caused them because days or weeks have passed. Cryptosporidium averages about 7 days but can take nearly a month. Giardia typically takes 1 to 4 weeks. The amoeba that causes dysentery (Entamoeba histolytica) can produce symptoms in as few as 2 to 3 days or as many as 4 weeks.
Trichinella from undercooked meat has one of the widest windows, from 1 to 2 days for initial gut symptoms to 2 to 8 weeks for the muscle pain and swelling that occur as larvae migrate through your body. This long, variable timeline means that by the time you feel sick, you may not remember the specific meal, swim, or soil contact that was responsible.
Who Faces the Highest Risk
Certain situations raise your exposure significantly. Traveling to tropical regions with limited sanitation infrastructure increases the odds of encountering soil-transmitted worms, waterborne parasites, and insect-borne diseases. Eating adventurously (raw fish, bushmeat, street food) adds risk wherever you are. Owning young pets that haven’t been dewormed, drinking untreated water from natural sources, and swimming in lakes or poorly maintained pools all create opportunities for parasites to enter your body.
Children, people with weakened immune systems, and pregnant women tend to experience more severe consequences from the same infections that a healthy adult might barely notice. Keeping hands clean, cooking meat thoroughly, treating or filtering water from uncertain sources, and wearing shoes in areas where sanitation is poor collectively eliminate the majority of common exposure routes.