A parasite is an organism that lives on or inside another living thing and survives at that host’s expense. What it does, in practical terms, is take resources, damage tissues, dodge the immune system, and in some remarkable cases, alter the host’s behavior to improve its own chances of survival. More than 1.4 billion people worldwide required treatment for parasitic neglected tropical diseases in 2023 alone, making parasites one of the most widespread health threats on the planet.
How Parasites Steal Nutrients
The most fundamental thing a parasite does is feed. Some parasites consume food directly from your gut before your body can absorb it. Others latch onto intestinal walls and feed on blood or tissue. The result is the same: your body gets less of what it needs.
Several common intestinal parasites cause significant malabsorption, meaning your digestive system loses its ability to pull nutrients from food efficiently. Giardia, for example, coats portions of the small intestine and interferes with the absorption of fats and vitamins. The fish tapeworm competes specifically for vitamin B12, potentially leading to a type of anemia that mimics B12 deficiency from diet alone. Hookworms attach to the intestinal lining and feed on blood, which over months can cause iron-deficiency anemia, especially in children. This nutrient theft is why unexplained weight loss, fatigue, and increased appetite are hallmark signs of parasitic infection.
Tissue Damage and Organ Involvement
Parasites don’t just sit quietly in one spot. Many species migrate through the body during their life cycle, physically tearing through tissue along the way. Roundworm larvae, for instance, travel from the intestines through the liver and lungs before returning to the gut to mature. Each stage of that journey causes inflammation and tissue damage.
Some parasites form cysts in organs. When the immune system can’t fully eliminate a larva, it walls the parasite off in a pocket of inflamed tissue called a granuloma. These cysts can develop in the liver, brain, eyes, or muscles depending on the species. In the brain, cysts from the pork tapeworm are one of the leading causes of seizures in parts of the world where the parasite is common. Research in animal models shows that without a strong antibody response, these tissue lesions grow larger and more destructive, with greater areas of dead tissue forming around trapped larvae.
Even parasites that stay in the intestines cause structural damage. Some create lesions in the intestinal lining, disrupting the barrier that normally keeps gut contents separate from the bloodstream. This can trigger chronic inflammation and make the gut more vulnerable to secondary infections.
Evading the Immune System
Your immune system is built to recognize and destroy foreign invaders, but parasites have evolved sophisticated ways to stay one step ahead. One of the most effective strategies is called antigenic variation. The malaria parasite, for example, carries a large library of genes that code for different surface proteins. At any given time, it only displays one version on its surface. By the time your immune system builds antibodies against that version, the parasite switches to a different one. This constant costume change means your body is always fighting the last version of the parasite rather than the current one.
Other parasites take a more passive approach. Some coat themselves in host molecules, essentially disguising themselves as the body’s own cells. Others suppress the immune response directly, dialing down inflammation in ways that let them persist for years. This is why certain parasitic infections become chronic. The parasite isn’t overwhelming your immune system with brute force. It’s quietly outsmarting it.
Changing How Hosts Behave
Perhaps the most unsettling thing parasites do is manipulate the behavior of their hosts. This isn’t science fiction. It’s a well-documented survival strategy that improves the parasite’s chances of completing its life cycle.
The most studied example is Toxoplasma gondii, a single-celled parasite that can only reproduce sexually inside cats. When it infects a rat, it embeds itself in the brain and causes the rat to become attracted to cat urine rather than afraid of it. Infected rats also show increased aggression and greater risk-taking. The result: the rat is more likely to be caught and eaten by a cat, delivering the parasite exactly where it needs to go.
Scientists have identified three main pathways parasites use to pull this off. Some alter communication between the immune system and the brain, using chronic inflammation in brain tissue to disrupt normal nerve signaling. Others release molecules that interact directly with the nervous system to change how neurons fire. A third route involves changing which genes are active in infected cells, reprogramming host biology at a fundamental level. The immune system, in some cases, acts as an intermediary: the parasite triggers an immune response that produces brain inflammation, and the inflammation itself changes behavior. This may actually be easier for the parasite than crossing the blood-brain barrier directly.
Common Symptoms of Infection
The specific symptoms depend on the type of parasite and where it lives in the body, but many infections share a recognizable pattern. Gastrointestinal parasites commonly cause diarrhea, nausea, vomiting, and abdominal pain. You may notice unexplained weight loss alongside an increased appetite, which reflects the parasite consuming nutrients your body would otherwise use.
Beyond the gut, parasitic infections often cause muscle aches, fatigue, weakness, and fever with chills. Skin involvement can show up as rashes, intense itching, or visible bite marks. Some parasites that affect the brain or nervous system cause sleep problems, and in severe cases, seizures or changes in cognition. Ectoparasites like lice or scabies mites may be visible on the skin, hair, or clothing during a physical exam.
How People Get Infected
Parasites reach new hosts through a few well-established routes. The fecal-oral pathway is among the most common: contaminated water or food carries parasite eggs or cysts into the digestive system. This is why soil-transmitted worm infections are concentrated in areas with limited sanitation. In 2023, roughly 876 million children aged 1 to 14 needed preventive treatment for soil-transmitted worms across 86 countries.
Some parasites enter through the skin. Hookworm larvae in contaminated soil can burrow directly through the soles of the feet. Schistosomiasis, which affected over 253 million people requiring treatment in 2023, spreads when larvae in freshwater penetrate the skin of anyone wading, swimming, or bathing in infected sources. Insect vectors are another major route: mosquitoes transmit malaria, sandflies carry leishmaniasis, and tsetse flies spread sleeping sickness. Direct contact, including skin-to-skin transmission of mites or lice, accounts for the rest.
How Parasitic Infections Are Treated
Antiparasitic medications work through three basic mechanisms. Some kill the parasite or its eggs outright. Others stop the parasite from growing or reproducing, giving your immune system time to clear the infection. A third category paralyzes the parasite so it can no longer grip the intestinal wall, allowing your body to flush it out naturally.
Which medication you receive depends entirely on the species involved. Identifying the parasite accurately matters because drugs effective against worms may do nothing against single-celled parasites, and vice versa. Microscopic examination of blood or stool samples remains the standard approach for many infections. For malaria specifically, examining blood smears under a microscope is still considered the gold standard, though rapid tests and DNA-based detection methods are available. A single negative blood smear doesn’t always rule out malaria. Samples are typically repeated every 12 to 24 hours for up to three sets before an infection can be confidently excluded.
Treatment for most common intestinal parasites is straightforward and effective when the infection is caught. Mass prevention programs deliver deworming medication to hundreds of millions of children each year in affected regions. In 2023, over 457 million children received preventive deworming treatment globally, and nearly 91 million people were treated for schistosomiasis.