You get parasites by swallowing their eggs or larvae (usually through contaminated food, water, or unwashed hands), by being bitten by an infected insect, or by direct skin contact with contaminated soil. Some parasites also spread from pets to people and from person to person through close household contact. The specific route depends on the type of parasite, but most infections trace back to one of these pathways.
Swallowing Contaminated Food or Water
The most common way people pick up intestinal parasites is the fecal-oral route. That sounds alarming, but it simply means that microscopic eggs or cysts from an infected person’s or animal’s stool end up in something you eat or drink. This can happen when produce is irrigated with contaminated water, when food is prepared by someone who didn’t wash their hands thoroughly, or when you drink untreated water from a stream, lake, or well.
Giardia is a classic example. Its tough cysts survive in water for weeks, which is why hikers and campers who drink from rivers get it so often. The infection typically shows up about a week after exposure and causes watery diarrhea, cramping, and nausea lasting one to three weeks. Cryptosporidium spreads the same way. Both parasites also cause outbreaks in daycare centers, where diaper changes and hand-to-mouth behavior create easy opportunities for the eggs or cysts to pass between toddlers and caregivers.
Entamoeba, the parasite behind amoebic dysentery, also spreads through contaminated food and water. In some parts of the world, crops fertilized with human waste (“night soil”) are a significant source. All three of these parasites can also spread through oral-anal sexual contact.
Undercooked or Raw Meat and Fish
Certain parasites live inside the muscle tissue of animals and only infect you when you eat meat that hasn’t been cooked to a high enough internal temperature. Trichinella is the best-known example. It’s found in undercooked pork and especially in wild game like bear, wild boar, fox, wolf, and walrus. Using a food thermometer is the only reliable way to confirm the meat is safe. Curing, drying, smoking, or microwaving meat does not consistently kill the larvae.
Freezing standard cuts of pork (less than six inches thick) at -15°C (5°F) for 20 days will kill Trichinella. But that trick doesn’t work for wild game, because some species of the parasite are freeze-resistant. Tapeworms are another concern in undercooked beef, pork, and freshwater fish. The larvae embed themselves in the animal’s tissue and develop into adult worms in your intestines after you eat them.
Walking Barefoot on Contaminated Soil
Some parasites don’t need you to swallow them at all. Hookworm larvae hatch in warm, moist soil where infected people have defecated, and they can actively penetrate your skin, usually through the soles of your feet. From there, they travel through your bloodstream to your lungs and eventually your intestines. The two main species that infect humans thrive in tropical and subtropical regions with poor sanitation. Anyone who walks barefoot or sits with bare skin on contaminated ground is at risk.
Globally, soil-transmitted parasites remain enormously common. The World Health Organization reported that more than 883 million children worldwide required preventive treatment for soil-transmitted worm infections in 2024, and only about 57% of them received it.
Insect Bites
Bloodsucking insects act as biological shuttles for certain parasites. The insect picks up the parasite during a blood meal from an infected person or animal, the parasite multiplies inside the insect, and then it gets injected into a new host with every subsequent bite. Once an insect becomes infectious, it can transmit the parasite for the rest of its life.
Mosquitoes are the most significant carriers. Different mosquito species transmit malaria and lymphatic filariasis (a parasitic infection that can cause severe swelling of the limbs). Other insect-borne parasites include those spread by sandflies (leishmaniasis), tsetse flies (sleeping sickness), and blackflies (river blindness). These infections are concentrated in tropical regions, but travelers can bring them home.
Pets and Other Animals
Dogs and cats carry several parasites that can jump to humans. You can pick them up by touching an infected animal, handling its feces, or contacting contaminated surfaces in your home or yard. The list of parasites shared between pets and people includes hookworms, roundworms, tapeworms, Giardia, Cryptosporidium, and Toxoplasma.
Toxoplasma is particularly associated with cats. It sheds in cat feces, so cleaning a litter box is the primary risk. Hookworm larvae from dog or cat stool in your yard can also penetrate bare skin, causing itchy, winding rashes. Tapeworms can spread to people (especially young children) who accidentally swallow an infected flea from a pet. Regular deworming of pets, prompt cleanup of animal waste, and handwashing after handling animals or their belongings substantially reduce the risk.
Person-to-Person Spread
Pinworms are the textbook example of direct person-to-person transmission. A female pinworm crawls out of the intestine at night and lays thousands of eggs around the skin near the anus. The itching this causes leads to scratching, which transfers eggs to fingers and under fingernails. From there, the eggs land on bedding, towels, doorknobs, and toys. Other household members touch those surfaces, then touch their mouths, and the cycle continues. It’s common for an entire family to become infected at once.
Pinworm is by far the most common parasite infection in the United States, especially among school-age children. The eggs are light enough to become airborne when you shake out bedding, which means you can even inhale and swallow them. Handwashing, keeping fingernails short, and laundering bedding in hot water are the main ways to break the cycle.
Who Faces the Highest Risk
Certain situations raise your chances significantly. Traveling to regions with limited water treatment and sanitation infrastructure puts you in contact with parasites your body has never encountered. Children in daycare settings face higher exposure because of frequent diaper changes and the tendency to put hands and objects in their mouths. People who work with soil, garden barefoot, or handle animals professionally encounter more opportunities for skin penetration or accidental ingestion.
Immune suppression also plays a role. The same parasite that causes mild symptoms in a healthy adult can cause severe, prolonged illness in someone with a weakened immune system. And because many parasites produce no symptoms for weeks or months, infections can go unrecognized and spread to others long before anyone realizes there’s a problem. Giardia, for instance, can take anywhere from 1 to 14 days after exposure before symptoms appear.