Parasites enter the human body through a surprisingly wide range of routes: contaminated food, untreated water, insect bites, soil contact, close contact with other people, and even your pets. Some are swallowed as microscopic eggs or cysts you can’t see or taste. Others burrow directly through your skin. Understanding the specific pathways helps you know where the real risks are and how to reduce them.
Undercooked or Raw Meat
Eating raw or undercooked meat is one of the most common ways people pick up parasites. Trichinella, the worm responsible for trichinosis, lives in the muscle tissue of meat-eating animals like bears, wild boar, walrus, and wild cats. Even tasting a small amount of raw meat during preparation is enough to cause infection. Homemade jerky and sausage made from wild game are also sources, because curing, salting, drying, smoking, or microwaving meat does not consistently kill the larvae.
Freezing works for commercial pork, but some Trichinella species found in wild game are freeze-resistant. Cooking meat to a safe internal temperature throughout remains the most reliable way to kill these parasites. Raw or undercooked freshwater fish can carry liver flukes and other worms, while raw seafood (think sushi-grade fish that hasn’t been properly frozen) can harbor Anisakis larvae, which cause stomach symptoms within hours of eating infected fish.
Contaminated Water
Cryptosporidium and Giardia are the two parasites most commonly spread through water. Cryptosporidium, often called “Crypto,” causes watery diarrhea and is extremely tolerant of chlorine, which means it can survive in treated swimming pools, splash pads, and water parks. A 2024 outbreak at a Georgia water park illustrated exactly this problem. Swallowing even a small mouthful of contaminated recreational water is enough.
Drinking water from untreated sources like streams, lakes, or wells in areas with poor sanitation carries similar risks. Municipal water systems in developed countries generally filter out these organisms, but outbreaks still happen. If you’re traveling in regions without reliable water treatment, using a filter rated for parasites or boiling water before drinking is the safest approach.
Walking Barefoot on Contaminated Soil
Hookworm is the classic example of a parasite you don’t eat or drink. Instead, its larvae live in warm, moist soil contaminated with human or animal feces. When you walk barefoot on that soil, the larvae actively penetrate the skin of your feet and work their way into your bloodstream. From there, they travel to the intestines.
There’s no direct person-to-person transmission. Hookworm eggs passed in feces need about three weeks to mature in the soil before they become infectious. This is why hookworm thrives in areas with warm climates and inadequate sanitation, where open defecation contaminates the ground. Other soil-transmitted worms, like roundworm and whipworm, follow a different path. Their eggs are swallowed after contaminated soil gets on hands, food, or surfaces. Children playing in dirt are especially vulnerable.
Insect Bites
Bloodsucking insects are major parasite delivery systems, particularly in tropical and subtropical regions. Mosquitoes are the most significant. Different mosquito species transmit malaria (the deadliest parasitic disease worldwide) and lymphatic filariasis, which causes severe swelling in the limbs.
Other insect vectors each carry their own parasites:
- Sandflies transmit leishmaniasis, which can cause skin sores or damage internal organs.
- Tsetse flies spread African sleeping sickness, found in sub-Saharan Africa.
- Triatomine bugs (sometimes called “kissing bugs”) carry Chagas disease, common in Latin America.
- Blackflies transmit river blindness, which affects the eyes and skin.
In each case, the insect picks up the parasite from an infected person or animal during a blood meal, the parasite multiplies inside the insect, and then gets injected into the next person bitten. Bed nets, insect repellent, and protective clothing are the primary defenses in areas where these insects are active.
Pets and Other Animals
Dogs and cats commonly carry roundworms, and their feces can contain eggs that survive in soil and on surfaces for long periods. Toxocariasis spreads when people, particularly young children, ingest contaminated dirt or touch contaminated surfaces and then put their hands in their mouths. In rare cases, these larvae migrate to the eyes or internal organs.
Cat feces also carry Toxoplasma, a parasite that’s a well-known risk during pregnancy. Cleaning a litter box, gardening in soil where cats have defecated, or handling anything contaminated with cat feces can lead to exposure. Regularly deworming pets and washing your hands thoroughly after handling litter or soil reduces this risk significantly.
Person-to-Person Contact
Some parasites spread directly between people. Pinworm is the most common example, especially among young children. A child scratches the itchy area around the anus (where female pinworms lay eggs at night), picks up microscopic eggs under the fingernails, and transfers them to toys, bedding, doorknobs, or directly to another child. The eggs are then swallowed, and the cycle continues. Pinworm infections can spread rapidly through households and classrooms.
Scabies, caused by tiny mites that burrow into the skin, spreads through prolonged skin-to-skin contact. Head lice transfer the same way. Both are common among children during play and in crowded living conditions. These parasites live on or just beneath the skin rather than inside the body, but they’re parasites all the same.
Unwashed Produce and Contaminated Surfaces
Fresh fruits and vegetables grown in or near contaminated soil or irrigated with untreated water can carry parasite eggs and cysts on their surface. Cyclospora, a parasite that causes prolonged watery diarrhea, has been linked to imported fresh produce like raspberries, basil, and lettuce. Washing produce under running water removes some surface contamination, though it doesn’t eliminate all parasites. Peeling fruits and cooking vegetables are more effective.
Contaminated surfaces matter too, particularly in childcare settings and institutions. Cryptosporidium cysts and pinworm eggs can persist on surfaces, and hand-to-mouth contact completes the transmission cycle. This is why handwashing with soap and water (not just hand sanitizer, which doesn’t kill most parasite eggs and cysts) is one of the single most effective prevention measures.
How Long Before You Notice Symptoms
The gap between swallowing or absorbing a parasite and feeling sick varies enormously depending on the species. Some parasites cause symptoms within hours. Anisakis from raw fish can trigger intense stomach pain the same day. Cryptosporidium typically causes diarrhea within five to seven days.
Many intestinal worms take longer. Hookworm symptoms appear days to weeks after skin penetration. Whipworm takes two to eight weeks. Schistosomiasis, picked up from freshwater snails in tropical lakes and rivers, typically causes symptoms four to eight weeks after exposure. And some parasites are remarkably slow: cystic echinococcosis (from dog feces carrying tapeworm eggs) can take years to produce symptoms as cysts grow slowly in the liver or lungs.
This wide range of timelines makes parasitic infections tricky to connect to a specific exposure, especially for travelers who may not develop symptoms until weeks or months after returning home.
How Parasitic Infections Are Detected
If your doctor suspects a parasitic infection, the most common first step is an ova and parasite test, which examines a stool sample under a microscope for eggs or parasites. Because parasites shed eggs intermittently, a single sample often isn’t enough. The CDC recommends submitting three or more stool samples collected on separate days to improve accuracy. Even then, a negative result doesn’t always rule out infection, and your doctor may send a repeat sample to a different lab for confirmation.
Some infections are detected through blood tests that look for immune responses to specific parasites, or through imaging when parasites form cysts in organs. Skin-based parasites like scabies are usually diagnosed by visual examination or skin scraping.