A parasitic infection is any illness caused by an organism that lives on or inside your body and survives by feeding off you. Parasites range from single-celled organisms too small to see to worms several feet long, and they affect hundreds of millions of people worldwide. In 2024 alone, malaria, just one parasitic disease, caused an estimated 282 million cases and 610,000 deaths globally.
Three Types of Parasites That Infect Humans
Parasites that cause disease in humans fall into three broad categories: single-celled organisms called protozoa, worms (helminths), and ectoparasites that live on the skin’s surface.
Protozoa are microscopic, single-celled parasites. What makes them dangerous is their ability to multiply inside your body, allowing a small initial infection to grow rapidly. The malaria parasite is a protozoan that replicates inside red blood cells. Giardia, another protozoan, colonizes the intestines after you swallow contaminated water. Toxoplasma, which spreads through undercooked meat and cat feces, is yet another.
Helminths are worms visible to the naked eye in their adult form. Unlike protozoa, adult helminths cannot multiply inside you. Instead, they produce eggs or larvae that leave the body and mature elsewhere before infecting a new host. There are three main groups: flatworms (including tapeworms and flukes), roundworms, and thorny-headed worms. Roundworms are especially versatile. Their adult forms can settle in the gut, blood, lymphatic system, or tissues just beneath the skin, and their larvae can cause damage by migrating through various organs.
Ectoparasites live on the outside of your body. Lice, fleas, ticks, and mites all fall into this category. Some are blood feeders: mosquitoes pierce capillaries directly, while other biting insects damage tissue and lap up the pooled blood. The jigger flea takes things further, burrowing into the skin and subcutaneous tissue to form painful nodules. Scabies mites tunnel into the skin as well, and they spread through direct skin-to-skin contact lasting about 15 to 20 minutes.
How Parasites Get Into Your Body
Parasites enter the body through several distinct routes, and knowing these pathways is the key to prevention.
Contaminated water and food are the most common routes for intestinal parasites. Swallowing untreated water carrying Giardia cysts or eating undercooked meat containing Toxoplasma cysts introduces parasites directly into the digestive tract. Raw or undercooked shellfish can also be a source, since parasites washed into seawater concentrate in oysters, mussels, and clams.
Insect vectors are the primary route for blood-borne parasites. Mosquitoes are the single most important disease vector, transmitting malaria, lymphatic filariasis, and other parasitic infections during blood feeding. Ticks rank second. For parasites like the malaria organism and filarial worms, the insect isn’t just a taxi. The parasite actually transforms inside the insect from a non-infective stage to an infective one before it can be transmitted during a bite.
Soil contact is another route. Walking barefoot on contaminated ground allows hookworm larvae to penetrate the skin. Gardening or playing in soil or sand where animal feces have been deposited can expose you to Toxoplasma and other parasites. Direct person-to-person contact spreads ectoparasites like lice and scabies mites, and pinworms pass easily among children through contaminated fingers and surfaces.
Common Symptoms
Symptoms depend heavily on where in your body the parasite has settled, which is why parasitic infections can mimic many other conditions.
Gastrointestinal parasites typically cause diarrhea, cramping, abdominal pain, gas, nausea, and vomiting. In heavy infections, worms can cause intestinal obstruction. Chronic infections with soil-transmitted worms lead to anemia and, in children, stunted growth and impaired cognitive development.
Parasites that travel beyond the gut produce a wider range of symptoms. Fever and muscle aches are common with blood-borne infections like malaria. Parasites affecting the nervous system can cause seizures, severe headaches, or disorientation. Skin parasites and those migrating through tissues produce redness, itching, rashes, or open sores. Lymphatic filariasis, spread by mosquitoes, causes dramatic swelling of the arms, legs, or scrotum as worms block the lymphatic drainage system. Onchocerciasis, a roundworm infection, causes intense itching and can impair vision.
Some parasitic infections produce no obvious symptoms for months or even years, particularly when the parasite load is low. This is one reason infections can go undiagnosed for long periods.
How Parasitic Infections Are Diagnosed
There is no single test that screens for all parasites. Diagnosis depends on the type of infection suspected.
For intestinal parasites, the standard approach is an ova and parasite test, which examines stool samples under a microscope for eggs or the parasites themselves. The CDC recommends collecting three or more stool samples on separate days, since parasites shed eggs intermittently and a single sample can miss them. If stool tests come back negative but symptoms persist, an endoscopy or colonoscopy allows direct visualization of the intestinal lining.
Blood-borne parasites like malaria are diagnosed with a blood smear: a drop of blood is spread on a slide, stained, and examined under a microscope. Serology tests detect antibodies your immune system produces in response to a parasite, which is useful for infections where the parasite itself is hard to find in a blood sample. Imaging scans (X-ray, MRI, or CT) help identify parasitic infections that form cysts or lesions in organs like the brain or liver.
Treatment Basics
Antiparasitic medications work by exploiting biological differences between the parasite and your own cells, which is why they can kill the invader without harming you. For worm infections, medications generally work in one of two ways. Some disrupt the worm’s internal cell structure or energy production, essentially starving it. Others paralyze the worm by interfering with its neuromuscular system, causing it to lose its grip on the intestinal wall so your body can expel it naturally. Some drugs cause rigid paralysis (the worm locks up), while others cause flaccid paralysis (the worm goes limp). Either way, the worm can no longer hold on.
Protozoan infections require different medications tailored to the specific organism. Malaria, for example, has its own class of drugs distinct from those used for intestinal protozoa like Giardia. Treatment duration varies widely. A pinworm infection may clear with a single dose plus a follow-up dose two weeks later, while other parasitic diseases require weeks of treatment.
Practical Prevention Steps
Most parasitic infections are preventable with basic precautions around food, water, and insect exposure.
Cook meat to safe internal temperatures: at least 145°F (63°C) for whole cuts of meat (with a three-minute rest before eating), 160°F (71°C) for ground meat, and 165°F (74°C) for all poultry. Use a food thermometer placed in the thickest part. Freezing meat at 0°F or below for several days before cooking also greatly reduces the risk. Rinse all fruits and vegetables under running water, avoid unpasteurized goat’s milk, and skip raw shellfish in areas where contamination is a concern.
Avoid drinking untreated water, whether from streams, lakes, or tap systems in regions where water treatment is unreliable. Wash cutting boards, utensils, and countertops with hot soapy water after preparing each food item to prevent cross-contamination.
For insect-borne parasites, use insect repellent, sleep under treated bed nets in malaria-endemic areas, and wear long sleeves and pants in tick-heavy environments. If you have cats, change the litter box daily. The Toxoplasma parasite doesn’t become infectious until one to five days after a cat sheds it, so daily cleaning removes it before it poses a risk. Wear gloves when gardening or handling soil that may be contaminated with animal feces, and wash your hands thoroughly afterward.
Who Is Most at Risk
Parasitic infections occur everywhere, but the burden falls disproportionately on tropical and subtropical regions where sanitation infrastructure is limited and insect vectors thrive. The WHO African Region accounted for 95% of all malaria cases and deaths in 2024, and children under five bore 76% of malaria deaths in that region.
Travelers to endemic areas face elevated risk, as do people with weakened immune systems, who are more vulnerable to severe outcomes from infections that would otherwise stay mild. Children are particularly susceptible to soil-transmitted worms and pinworms, both because of hand-to-mouth behavior and because chronic infection during childhood can permanently affect growth and cognitive development.