A parasite is an organism that lives on or inside another organism, called a host, and survives by feeding off that host at the host’s expense. This relationship, called parasitism, is distinct from other biological partnerships: in mutualism, both organisms benefit, and in commensalism, one benefits while the other is unaffected. Parasites always cause some degree of harm. They range from microscopic single-celled organisms to worms visible to the naked eye to insects that burrow into skin.
The Three Main Types of Parasites
Parasites that infect humans fall into three broad categories: protozoa, helminths, and ectoparasites. Each group differs dramatically in size, complexity, and how it interacts with the human body.
Protozoa
Protozoa are single-celled organisms, invisible without a microscope. Despite being just one cell, they carry out surprisingly complex functions and can multiply rapidly inside a human host. That ability to reproduce within the body is what makes protozoan infections so dangerous: a tiny initial exposure can balloon into a massive infection. The malaria parasite, for instance, can establish an infection from fewer than ten organisms delivered by a single mosquito bite. Once inside the bloodstream, it hijacks red blood cells and replicates through repeated cycles until it infects 10 percent or more of all red blood cells, reaching roughly 400 million parasites per milliliter of blood.
Other well-known protozoa include Giardia, which infects the intestines and causes severe diarrhea, and Toxoplasma, found in undercooked meat, cat feces, and contaminated soil or water. Many protozoa survive outside the body by forming tough protective shells called cysts, which allow them to persist in water or soil until a new host ingests them.
Helminths
Helminths are parasitic worms, and they come in three main varieties: flatworms (including flukes and tapeworms) and roundworms.
Flukes are leaf-shaped flatworms that use suction cups around the mouth and belly to grip onto host tissues. Most flukes carry both male and female reproductive organs in the same body, with the notable exception of blood flukes (schistosomes), which have separate sexes. These worms typically infect the liver, lungs, or blood vessels.
Tapeworms are elongated, segmented flatworms that live in the intestines. Their bodies are made up of repeating segments called proglottids, each containing a complete set of reproductive organs. Tapeworms have no digestive tract at all. Instead, they absorb nutrients directly through their body wall from the digested food passing through the host’s intestine.
Roundworms, or nematodes, are cylindrical rather than flat and have a complete digestive system with both a mouth and an anus. Males are typically smaller than females and have a curved tail. Roundworms include hookworms, which enter through the skin of bare feet, and Ascaris, one of the most common human parasites worldwide.
Ectoparasites
Ectoparasites live on the outside of the body rather than inside it. Ticks, fleas, lice, mites, and bedbugs all fall into this category. They attach to or burrow into the skin and can remain there for weeks or months, feeding on blood or skin cells. Beyond the direct irritation they cause, many ectoparasites also transmit other infections while feeding. Ticks, for example, can carry bacteria that cause Lyme disease, and certain flea species can spread plague.
Scabies is caused by tiny mites that burrow into the upper layers of skin, triggering an intensely itchy rash with small bumps or blisters. Head lice and body lice are another common ectoparasite, clinging to hair or clothing and laying eggs (nits) that cement to individual strands.
How Parasites Spread
Parasites reach new hosts through several routes. The most common is the fecal-oral route: a person swallows food or water contaminated with parasite eggs or cysts shed in another person’s or animal’s feces. This is how Giardia, Cryptosporidium, and many intestinal worms spread, and it’s especially common in areas with poor sanitation or untreated water supplies.
Vector-borne transmission happens when a biting insect delivers parasites directly into the bloodstream. Mosquitoes transmit malaria this way. Ticks, sandflies, and certain freshwater snails also serve as vectors for different parasitic diseases.
Some parasites enter through the skin without any insect intermediary. Hookworm larvae in contaminated soil can penetrate the soles of bare feet. Others, like Toxoplasma, can pass from a newly infected pregnant woman to her fetus, potentially causing severe brain and eye damage. And eating raw or undercooked meat, particularly pork, wild game, or freshwater fish, is a well-known route for tapeworm and fluke infections.
Symptoms of Parasitic Infections
What a parasitic infection feels like depends entirely on what type of parasite is involved and where it settles in the body. Intestinal parasites commonly cause diarrhea, cramping, gas, nausea, and vomiting. Parasites that enter the bloodstream or organs tend to produce more systemic symptoms: fever, muscle aches, and fatigue. Some infections cause skin-specific problems like redness, intense itching, rashes, or open sores.
In more severe cases, parasites that reach the brain or nervous system can cause seizures, disorientation, or severe headaches. Many parasitic infections are mild or even symptom-free for long periods, which is part of what makes them so effective. A person can carry and spread certain parasites for months or years without realizing they’re infected.
How Parasitic Infections Are Detected
The most common test for intestinal parasites is a stool sample, sometimes called an ova and parasite test. A lab technician examines the sample under a microscope looking for eggs or whole organisms. Because parasites aren’t always shed consistently, the CDC recommends submitting three or more samples collected on separate days to improve accuracy.
Blood tests can detect certain parasites that live in the bloodstream. A blood smear, where a drop of blood is stained and examined under a microscope, can reveal malaria parasites inside red blood cells. Antibody tests check whether your immune system has mounted a response against a specific parasite, which is useful for infections that don’t show up easily in stool or blood smears.
When stool tests come back negative but symptoms persist, doctors may use endoscopy or colonoscopy to look directly at the intestinal lining. Imaging scans like MRI or CT can detect damage or cysts that parasites have caused in organs like the liver, lungs, or brain.
Global Scale of Parasitic Disease
Parasitic infections are among the most widespread health problems on the planet. According to the World Health Organization, an estimated 1.495 billion people required treatment for neglected tropical diseases in 2023, many of which are caused by parasites. That number has been declining steadily, down 32% from 2010 levels, largely thanks to mass treatment programs. In 2023 alone, over 867 million people received treatment for at least one neglected tropical disease.
The burden falls disproportionately on tropical and subtropical regions with limited access to clean water, sanitation, and healthcare. Soil-transmitted helminths alone infect hundreds of millions of people, particularly children, contributing to malnutrition, anemia, and impaired growth.
Practical Prevention
Most parasitic infections are preventable with basic precautions. Washing your hands thoroughly with soap before meals and after using the bathroom is the single most effective step against fecal-oral transmission. When traveling in areas with questionable water quality, drink only bottled, boiled, or chemically treated water and avoid raw vegetables or salads that may have been washed in untreated water. Cooking meat to safe internal temperatures kills tapeworm cysts, Toxoplasma, and other foodborne parasites.
To prevent vector-borne parasites, use insect repellent, wear long sleeves and pants in high-risk areas, and sleep under bed nets where malaria is present. Walking barefoot on soil or sand in tropical regions puts you at risk for hookworm and other skin-penetrating parasites, so wearing shoes is a simple but important habit. If you have cats, cleaning litter boxes daily reduces the risk of Toxoplasma exposure, since the parasite’s cysts need at least 24 hours after being shed to become infectious.