More than 300 species of parasites can infect humans, and they fall into three broad categories: single-celled organisms that live in your blood or gut, parasitic worms that settle into your intestines or other organs, and creatures that attach to your skin or burrow into it. Globally, an estimated 1.5 billion people are infected with just three species of soil-transmitted worms alone, making parasitic infections one of the most common health problems worldwide.
Three Categories of Human Parasites
Every parasite that lives in or on a human body belongs to one of three groups. Protozoa are single-celled organisms too small to see without a microscope. They typically live in your intestines, blood, or tissues. Helminths are parasitic worms, ranging from tiny threadlike creatures to tapeworms that can grow several feet long, and they most often inhabit the digestive tract. Ectoparasites live on the outside of your body, attaching to or burrowing into the skin to feed on blood or tissue.
Protozoa: Single-Celled Parasites
Protozoa cause some of the most well-known parasitic diseases. They spread through contaminated water, insect bites, sexual contact, or the fecal-oral route, where microscopic amounts of infected stool end up in food, water, or soil that another person ingests.
The protozoan behind malaria, transmitted by infected mosquitoes, remains one of the deadliest parasites on Earth. Once inside the body, it invades red blood cells and causes cycles of fever, chills, headache, muscle pain, and vomiting. Giardia is one of the most common waterborne parasites in the United States, causing watery diarrhea, cramping, and gas after someone swallows contaminated water from lakes, streams, or poorly treated supplies. Cryptosporidium spreads the same way and produces similar symptoms.
Chagas disease, caused by a protozoan spread through the feces of “kissing bugs,” affects millions of people in Central and South America. The bug bites the skin, then defecates near the wound. When the person scratches the bite, the parasite enters the body. In 20 to 30 percent of infected people, the infection eventually causes serious inflammation of the heart muscle. Trichomoniasis, one of the most common sexually transmitted infections, is caused by a protozoan that infects the genital tract. Women often notice itching, burning, and a fishy-smelling discharge, while men may experience irritation inside the penis and burning after urination.
In the U.S., the most frequently diagnosed intestinal protozoa include Giardia, Cryptosporidium, and the amoeba that causes amebic dysentery.
Helminths: Parasitic Worms
Parasitic worms are grouped into three types based on their shape: roundworms, tapeworms, and flukes. Each has a distinct body plan and lifecycle, but all depend on a human host for at least part of their development.
Roundworms
Roundworms are cylindrical, thread-shaped worms. The large intestinal roundworm is the most common helminth infection in the world, part of a group of soil-transmitted worms that collectively infect about 1.5 billion people, roughly 24 percent of the global population. Eggs are shed in human stool, contaminate soil in areas with poor sanitation, and are swallowed when people eat unwashed produce or touch contaminated dirt and then their mouths.
Pinworm is the most common worm infection in the United States, affecting an estimated 40 million Americans. It spreads easily among children in households and schools. Female pinworms crawl out of the intestine at night to lay eggs around the anus, causing intense itching. Scratching transfers microscopic eggs to fingers, bedding, and surfaces, where other people pick them up.
Hookworms have one of the more dramatic lifecycles. Larvae living in contaminated soil penetrate bare feet, enter the bloodstream, travel to the heart, then the lungs. From the lungs they crawl up the airways into the throat, get swallowed, and finally settle in the small intestine where they mature and feed on blood. Whipworm, the third major soil-transmitted species, lives in the large intestine and can cause chronic diarrhea and abdominal pain in heavy infections. Over 600 million people are also estimated to carry threadworm, another soil-transmitted species whose distribution overlaps heavily with these other worms.
Tapeworms
Tapeworms are flat, segmented worms that live in the intestinal tract. People get them by eating undercooked beef, pork, or freshwater fish containing larval cysts. A beef tapeworm can grow to 25 feet or more inside the intestine. Most infections cause mild or no symptoms, though some people notice segments of the worm in their stool. The pork tapeworm poses a more serious risk: if someone swallows its eggs instead of its larvae, the larvae can form cysts in the brain, muscles, or other organs, a condition the WHO lists as a neglected tropical disease.
Flukes
Flukes are leaf-shaped flatworms that use suckers to attach inside their host. Their lifecycles always involve snails as an intermediate host. Humans pick up most flukes by eating raw freshwater fish, watercress, or other aquatic plants carrying the larvae. Different species target different organs: liver flukes settle in the bile ducts, intestinal flukes attach to the intestinal wall, lung flukes lodge in the lungs and can cause a chronic cough, and blood flukes cause schistosomiasis, a disease affecting hundreds of millions of people in tropical regions. Blood flukes are unusual among flukes because they have separate male and female worms rather than being hermaphroditic.
Ectoparasites: Skin-Dwelling Parasites
Ectoparasites live on or just beneath the surface of the skin. Three species of human lice are the most familiar examples. Head lice infest the scalp and are extremely common among school-age children. Body lice live in clothing and move onto the skin to feed. Pubic lice infest the groin area. All three are blood-sucking insects spread through close physical contact or shared personal items.
Scabies mites burrow into the upper layer of skin, where female mites lay eggs. The body’s immune reaction to the mites and their waste produces intense itching, especially at night, along with a rash of tiny bumps or tracks. Scabies spreads through prolonged skin-to-skin contact and is common in crowded living conditions. The WHO considers it a neglected tropical disease of public health importance.
Sand fleas cause a condition called tungiasis. The female flea burrows into the skin, usually around the toes or soles of the feet, and swells as her eggs develop. Ticks attach to the skin and feed on blood for days, and while they aren’t permanent residents, they can transmit other parasitic organisms during feeding. Animal hookworm larvae can also accidentally penetrate human skin, creating itchy, winding red tracks as the larvae migrate through the upper skin layer before dying off after weeks to months.
How Parasites Get Into Your Body
Parasites use a handful of reliable transmission routes. The fecal-oral route is the most common: an infected person’s stool contaminates soil, water, or food, and another person unknowingly ingests the eggs or cysts. This is how roundworms, whipworms, Giardia, and Cryptosporidium spread, and it’s the reason these infections concentrate in regions with limited sanitation.
Vector-borne transmission relies on biting insects. Mosquitoes carry malaria parasites. Kissing bugs transmit Chagas disease. Blackflies and other biting insects spread filarial worms that cause diseases like river blindness and elephantiasis. Skin penetration is the hookworm’s strategy: larvae in soil bore directly through the feet. Some parasites, like the one causing trichomoniasis, pass between people through sexual contact. And eating raw or undercooked meat or fish is the primary route for tapeworms and several species of flukes.
How Parasitic Infections Are Detected
The most common test for intestinal parasites is a stool exam, sometimes called an ova and parasite test. A lab technician examines stool samples under a microscope looking for eggs, larvae, or the parasites themselves. Because parasites shed eggs intermittently, the CDC recommends submitting at least three stool samples collected on separate days for reliable results.
Blood tests take two forms. A blood smear, examined under a microscope, can reveal parasites that live in the bloodstream, such as malaria or filarial worms. Antibody tests detect your immune system’s response to a parasite, which is useful for infections buried deep in tissues where direct sampling isn’t practical. For parasites that form cysts or cause damage in organs, imaging with X-ray, MRI, or CT scans can reveal lesions. In some cases, endoscopy allows a doctor to look directly at the intestinal lining to spot parasites or the damage they cause.