You can get a parasite by swallowing contaminated food or water, walking barefoot on infected soil, being bitten by certain insects, swimming in contaminated recreational water, or through contact with infected animals. Some parasites need to be eaten to infect you, while others can burrow directly through your skin. The specific route depends on the type of parasite, but most infections trace back to one of a handful of well-understood pathways.
Contaminated Food and Water
The most common route for parasitic infection worldwide is swallowing something contaminated with microscopic parasite eggs, cysts, or larvae. This is called fecal-oral transmission, and it works exactly how it sounds: an infected person or animal passes parasite eggs in their stool, those eggs end up in water or on food, and someone else unknowingly swallows them. Parasites like Giardia and Cryptosporidium spread this way through untreated drinking water, unwashed produce, or food handled by someone with an infection who didn’t wash their hands.
Undercooked or raw meat is another major source. Pork, wild game, and certain fish can harbor parasite larvae that survive unless the meat reaches a high enough internal temperature. Cooking all meats, especially wild game, to at least 160°F kills the larvae responsible for infections like trichinellosis. Toxoplasma, a parasite commonly associated with cats, also spreads through undercooked lamb, pork, and venison. Freezing meat can reduce risk for some parasites, but it isn’t reliable for all species.
Recreational and Drinking Water
Swimming pools, water parks, lakes, and rivers all pose a risk for certain parasites. Cryptosporidium is the biggest concern in treated recreational water because it is extremely tolerant of chlorine. Standard pool disinfection doesn’t kill it. In 2018, a Georgia water park had to shut down multiple times after a Cryptosporidium outbreak spread among visitors, causing diarrhea lasting two or more days. The outbreak continued in part because infected people kept using the facility while still contagious.
Swallowing even a small amount of water while swimming is enough to become infected. Lakes and streams carry additional risks because they can be contaminated by animal waste. Untreated well water and water from springs that haven’t been tested are also potential sources, particularly in rural areas.
Walking Barefoot on Contaminated Soil
Not all parasites need to be swallowed. Hookworm larvae live in warm, moist soil and can penetrate the skin of your feet when you walk barefoot. Once through the skin, they travel through the bloodstream to the intestines, where they mature into adult worms. An estimated 1.5 billion people globally, roughly 24% of the world’s population, are infected with soil-transmitted parasites including hookworms, roundworms, and whipworms. These infections are concentrated in tropical and subtropical regions with poor sanitation, but hookworm was historically common in the southeastern United States and still occurs there sporadically.
One hookworm species, Ancylostoma duodenale, can also spread through eating the larvae, but skin penetration through bare feet remains the primary route. Strongyloides, another soil-dwelling parasite, enters the body the same way. Wearing shoes in areas where soil may be contaminated with human or animal waste is the simplest prevention.
Insect Bites
Certain blood-feeding insects carry parasites and inject them directly into your bloodstream during a bite. Malaria, the most well-known parasitic disease, spreads through the bite of Anopheles mosquitoes. The parasite enters through the mosquito’s saliva as it feeds. Other vector-borne parasitic diseases include:
- Chagas disease: spread by triatomine bugs (sometimes called “kissing bugs”), which deposit parasite-laden feces near the bite wound rather than injecting through saliva
- Babesiosis: spread by hard-bodied ticks, primarily in the northeastern and upper midwestern United States
- Leishmaniasis: spread by sand flies, mainly in tropical and subtropical regions
- African sleeping sickness: spread by tsetse flies in sub-Saharan Africa
In the United States, the risk of catching most vector-borne parasites is very low, with the exception of babesiosis. Travelers to tropical regions face significantly higher exposure, particularly to malaria.
Pets and Other Animals
Dogs and cats can pass certain parasites to their owners. Toxocara roundworms are one of the most common examples. Infected dogs and cats shed roundworm eggs in their feces. Those eggs can survive in soil for months or years, and you can pick them up by touching contaminated dirt, a sandbox, or your pet’s fur and then touching your mouth. Children are especially vulnerable because they play in dirt and are less consistent about handwashing.
Cat litter boxes are a known source of Toxoplasma. Cats shed this parasite in their stool for a few weeks after becoming infected, and the eggs become infectious after sitting for one to five days. Cleaning the litter box daily reduces the risk significantly because the eggs haven’t had time to mature. Pet hookworm larvae can also penetrate human skin, causing an itchy, winding rash called cutaneous larva migrans, typically on the feet or legs after contact with contaminated sand or soil.
Person-to-Person Spread
Some parasites pass easily between people in the same household or childcare setting. Pinworms are the classic example. A female pinworm crawls out of the intestine at night to lay eggs around the anus, starting about five weeks after the initial infection. The eggs stick to fingers, bedding, clothing, and bathroom surfaces. Scratching the area transfers eggs to the hands, and from there to anything you touch. Other household members swallow the eggs and the cycle continues. Pinworm is extremely common in school-age children and spreads readily in families and daycare centers.
Giardia and Cryptosporidium can also pass between people through direct contact, particularly in settings involving diaper changes or caring for someone with diarrhea. Thorough handwashing with soap and water is the most effective way to break the cycle for all of these parasites. Hand sanitizer alone doesn’t reliably kill Cryptosporidium.
Who Is Most at Risk
Your risk depends heavily on where you live, what you eat, and your daily habits. People in tropical regions with limited sanitation infrastructure face the highest burden of soil-transmitted and waterborne parasites. Travelers to these areas are at elevated risk, especially if they drink untreated water or eat raw produce washed in local water.
In the United States and other high-income countries, the most common parasitic infections tend to be pinworm (especially in children), Giardia and Cryptosporidium (from contaminated water or person-to-person spread), and Toxoplasma (from undercooked meat or cat litter). People with weakened immune systems face more severe illness from many of these parasites, particularly Cryptosporidium, which can cause prolonged and dangerous diarrhea in immunocompromised individuals.
Outdoor enthusiasts who drink from streams without filtering, hunters who eat undercooked wild game, and people who garden or work in soil without gloves all have above-average exposure. Simple precautions, like filtering or boiling water, cooking meat thoroughly, wearing shoes outdoors, and washing hands after handling soil or animals, eliminate the vast majority of risk.