Parasites are organisms that live on or inside the human body, feeding off their host to survive. They range from single-celled organisms too small to see without a microscope to worms that can grow several feet long. An estimated 1.5 billion people worldwide, roughly 24% of the global population, are infected with soil-transmitted parasitic worms alone. Parasites fall into three main categories: protozoa, helminths, and ectoparasites.
Protozoa: Single-Celled Parasites
Protozoa are microscopic, one-celled organisms that can multiply inside the human body. This ability to reproduce in their host is what makes them particularly dangerous: a serious infection can develop from exposure to just a single organism. Some of the most well-known human diseases are caused by protozoa.
Malaria, caused by a protozoan spread through mosquito bites, remains one of the deadliest parasitic diseases on the planet. The World Health Organization estimated 263 million cases and 597,000 deaths from malaria in 2023 alone. Other common protozoan infections include giardia (spread through contaminated water, causing severe diarrhea) and cryptosporidium, which can survive for days even in properly chlorinated swimming pools.
Helminths: Parasitic Worms
Helminths are multicellular worms, often visible to the naked eye in their adult stage. Unlike protozoa, adult helminths cannot multiply inside the human body. Instead, they produce eggs that leave the body and develop in the environment before infecting a new host. There are two major groups: roundworms and flatworms.
Roundworms are among the most common intestinal parasites worldwide. Some species stay in the gut, while others migrate through the body. Trichinella, for example, is a roundworm acquired from undercooked meat that encysts in muscle tissue. Toxocara, picked up from contact with contaminated soil or animal feces, can migrate to the eyes or internal organs, potentially causing vision loss, fatigue, and abdominal pain.
Flatworms include two important subgroups. Flukes attach to the inner walls of the intestines, lungs, liver, or large blood vessels. Tapeworms typically live in the intestines, but their larval forms can migrate to other tissues. The pork tapeworm is especially concerning because its larvae can leave the intestine and colonize the central nervous system, a condition that can cause seizures.
Ectoparasites: Skin-Dwelling Parasites
Ectoparasites are insects and arachnids that attach to or burrow into the skin and remain there for weeks to months. The most common examples are lice, ticks, fleas, and mites. Beyond the direct irritation they cause, some ectoparasites also transmit other infections. Ticks, for instance, can carry bacteria that cause Lyme disease.
Scabies, caused by a tiny mite that burrows into the skin, is one of the most widespread ectoparasitic infections. The hallmark symptom is intense itching, though this doesn’t begin right away. The first time someone gets scabies, it takes several weeks for the body to develop a reaction. With a second infestation, itching can start within 24 hours. People with weakened immune systems can develop a more aggressive form called crusted scabies, which spreads more easily than ordinary scabies.
Common Symptoms
Parasitic infections produce a wide range of symptoms depending on where in the body the parasite lives. The most common presentation is intestinal illness: diarrhea, vomiting, cramping, and gas. These symptoms overlap with many other conditions, which is part of what makes parasitic infections tricky to identify.
Skin symptoms are also frequent, including redness, itching, rashes, or open sores. Some parasites cause fever and fatigue. In more serious cases, parasites that migrate beyond the gut to the brain, lungs, or liver can cause neurological symptoms, breathing problems, or organ damage. Helminths are the most likely to travel outside the intestinal tract, though certain protozoa can infect the brain as well.
How Parasitic Infections Are Diagnosed
The most common diagnostic tool is a stool test, formally called an ova and parasite exam. It looks for eggs or whole organisms in a stool sample. Because parasites shed eggs intermittently, the CDC recommends collecting three or more samples on separate days to improve accuracy.
When stool tests come back negative but symptoms persist, a doctor may use an endoscopy or colonoscopy to directly examine the intestinal lining. Blood tests serve two purposes: a serology test detects antibodies your immune system produces in response to a parasite, while a blood smear involves examining a drop of blood under a microscope to spot parasites like malaria that live in the bloodstream. For parasites that form cysts or lesions in organs, imaging scans like X-rays, MRIs, or CT scans can reveal the damage.
How Parasites Are Treated
Treatment depends on the type of parasite. Antiprotozoal drugs target single-celled parasites, including the specialized antimalarial medications used for malaria. Antihelminthic drugs treat worm infections. Ectoparasiticides, usually topical creams or lotions, kill lice, scabies mites, and similar skin parasites.
These medications work through several mechanisms. Some kill the parasite or its eggs directly. Others stop the parasite from growing or reproducing. A third approach paralyzes the worms so they can no longer grip the intestinal wall, allowing the body to flush them out naturally. Most intestinal parasitic infections clear with a short course of oral medication, though tissue-invading parasites may require longer or more intensive treatment.
How to Reduce Your Risk
Most parasitic infections enter the body through contaminated food, water, or soil. Handwashing with soap and water before eating, after using the bathroom, and after contact with animals is the single most effective preventive measure. Alcohol-based hand sanitizer (60% alcohol or higher) works as a backup, but it does not reliably remove all parasites. Cryptosporidium, for instance, resists hand sanitizer.
Food safety matters enormously, especially when traveling. Fully cooked foods served hot are safest. Raw or undercooked meat, fish, shellfish, and eggs all carry risk. Unpasteurized milk and dairy products can also harbor parasites. Fruits are safest when you rinse them with clean water and peel them yourself. Vegetables should be rinsed with safe water before cooking.
Water is a major transmission route. In areas where tap water may be unsafe, use commercially bottled water from a sealed container for drinking, cooking, brushing teeth, and making ice. Beverages made with freshly boiled water, like tea or coffee, are generally safe. Be cautious with ice in drinks, since it may be made from unfiltered local water. When swimming, avoid freshwater lakes and streams in regions where schistosomiasis is endemic, particularly parts of Africa, Asia, the Caribbean, and South America. In warm freshwater anywhere, wearing a nose clip helps prevent a rare but deadly brain-infecting amoeba from entering through the nasal passages.