A parasitic infection happens when an organism lives on or inside your body and feeds at your expense. These infections range from mild nuisances like head lice to life-threatening diseases like malaria, and they affect hundreds of millions of people worldwide. The organisms responsible fall into three broad categories, each causing distinct problems and requiring different treatments.
Three Types of Parasites
Parasites that infect humans come in three main groups: single-celled organisms called protozoa, parasitic worms called helminths, and ectoparasites that live on the skin’s surface.
Protozoa are microscopic, single-celled parasites that can infect your blood, gut, brain, skin, and eyes. Because they’re so small and reproduce quickly inside the body, even a tiny exposure can lead to a significant infection. Common protozoan infections include giardiasis (from contaminated water), cryptosporidiosis, trichomoniasis (a sexually transmitted infection), and malaria (spread by mosquitoes). Toxoplasma, another protozoan, is particularly dangerous during pregnancy and for people with weakened immune systems because it can infect the central nervous system.
Helminths are parasitic worms visible to the naked eye in their adult form. They come in three sub-groups. Flukes (trematodes) are leaf-shaped flatworms that attach to host tissues using oral and ventral suckers. Tapeworms (cestodes) are long, flat worms that live in the intestines and lack their own digestive tract entirely, absorbing nutrients directly through their outer surface. Roundworms (nematodes) are cylindrical, thread-like worms with a complete digestive system of their own. Hookworms, pinworms, and the worms that cause river blindness all belong to this group.
Ectoparasites live on the outside of your body rather than inside it. Head lice, pubic lice, and the mites that cause scabies are the most common examples. Head lice is actually the most common human ectoparasitic infection.
How You Get a Parasitic Infection
The route depends on the parasite. Many intestinal parasites spread through the fecal-oral route, meaning you ingest microscopic eggs or cysts from contaminated water, unwashed produce, or surfaces touched by someone who’s infected. This is how giardia and cryptosporidium typically spread.
Some parasites enter through the skin. Hookworm larvae in contaminated soil can burrow through bare feet. Schistosomiasis spreads through freshwater exposure when larvae released by snails penetrate the skin during swimming or wading.
Vector-borne parasites rely on biting insects. Mosquitoes transmit malaria. Tsetse flies spread African trypanosomiasis (sleeping sickness). Ticks carry babesiosis. Triatomine bugs, sometimes called “kissing bugs,” transmit Chagas disease.
Ectoparasites like lice spread through close physical contact or sharing personal items like hats and brushes. Scabies mites transfer through prolonged skin-to-skin contact.
Common Symptoms
Parasitic infections often cause intestinal symptoms: diarrhea, nausea, vomiting, bloating, and abdominal pain. These are especially common with protozoan gut infections like giardiasis and with intestinal worms like hookworms and tapeworms.
But symptoms aren’t limited to the digestive system. Parasites can also cause fever, fatigue, itchy skin rashes, and weight loss. Some infections affect the brain and lungs. Cysticercosis, caused by tapeworm larvae migrating to the brain, can trigger seizures. Certain roundworm larvae travel through the lungs and cause coughing or wheezing before settling in the intestines.
Many parasitic infections produce no obvious symptoms for weeks, months, or even years. Chagas disease can remain silent for decades before causing serious heart damage. This long delay between infection and symptoms is one reason parasitic diseases are so often underdiagnosed.
Who Is Most at Risk
Parasitic infections are most common in tropical and subtropical regions where sanitation infrastructure is limited and insect vectors thrive. In 2023, roughly 876 million children across 86 countries needed preventive treatment for soil-transmitted worms alone. Of those, about 71% were school-aged children between 5 and 14.
Many of the most damaging parasitic diseases fall under the World Health Organization’s list of neglected tropical diseases, including Chagas disease, leishmaniasis, schistosomiasis, lymphatic filariasis, and sleeping sickness. These diseases disproportionately affect the world’s poorest communities.
Travelers to endemic areas, people with compromised immune systems, young children, and pregnant women face elevated risk. In higher-income countries, pinworms remain common in school-age children, and giardia outbreaks occur periodically through contaminated water supplies.
How Parasitic Infections Are Diagnosed
Diagnosis usually starts with a stool sample. For intestinal parasites, lab technicians examine feces under a microscope looking for eggs, larvae, or worm segments. Multiple samples collected on different days improve accuracy, since parasites don’t always shed consistently.
Blood tests play several roles. A high count of a specific white blood cell type called eosinophils can signal a parasitic infection, particularly with worms. Antibody tests detect your immune system’s response to specific parasites. These are especially useful for infections that don’t show up in stool, like toxoplasmosis or cysticercosis.
Molecular testing using DNA-based methods has become increasingly important since the 1990s, particularly for distinguishing between closely related species that look identical under a microscope but differ in how dangerous they are. In some cases, doctors may use endoscopy or tissue biopsy to identify parasites directly in affected organs.
Treatment Options
Treatment depends entirely on the type of parasite involved. Antiparasitic medications fall into distinct classes, each targeting a specific group.
Protozoan infections are treated with antiprotozoal drugs. The specific medication varies by species: malaria, giardia, amoebic infections, and trichomoniasis each require different agents. Some protozoan infections, like Chagas disease, are much harder to treat in their chronic stage than when caught early.
Worm infections are treated with antihelminthic drugs. Some target tapeworms, others target roundworms, and a separate class works against flukes. Large-scale prevention programs in endemic countries distribute deworming medication to children. In 2023, about 457 million children in 53 countries received preventive deworming treatment.
Ectoparasites like lice and scabies are treated with topical creams, lotions, or shampoos applied directly to the skin. Most intestinal parasitic infections clear completely with a single course of treatment, though some require follow-up testing to confirm the parasite is gone.
Prevention Strategies
For travelers and anyone in areas where water safety is uncertain, the single most important step is controlling what you drink and eat. Use commercially bottled water from factory-sealed containers, or properly disinfect water by boiling it. Beverages made with water that has just been boiled, like tea and coffee, are generally safe. Cloudy or discolored water may contain chemical contaminants that boiling won’t remove, so stick to bottled water in those situations.
With food, fully cooked dishes served hot are the safest option. Perishable food should stay at refrigerator temperature (40°F / 4°C or below) and shouldn’t sit at room temperature for more than two hours. Wash all produce thoroughly with safe water, and peel fruits and vegetables when possible.
Beyond food and water, wear shoes in areas where soil may be contaminated to prevent hookworm. Use insect repellent and bed nets in regions where mosquitoes, tsetse flies, or other vectors transmit parasites. Avoid swimming in freshwater in areas where schistosomiasis is known to occur. For ectoparasites, avoid sharing personal items like combs, hats, and bedding with someone who has an active infestation, and wash potentially contaminated linens in hot water.