Parasites damage the human body through an extraordinary range of mechanisms: they bore through skin, feed on blood and tissue, form cysts in vital organs, steal nutrients, manipulate the immune system, alter brain chemistry, and in some cases promote cancer. Hundreds of species infect humans, from single-celled protozoa to meter-long tapeworms to mites that burrow into the outer layer of skin. The specific harm depends on the parasite, its life stage, and where in the body it ends up, but the cumulative picture is one of organisms that have evolved over millennia to exploit nearly every organ system we have.
How Parasites Break In
Many parasites enter the body through the mouth, riding in on contaminated food or water. But some take a more dramatic route, punching directly through the skin. Hookworm larvae and threadworm larvae (Strongyloides) are among the best-studied skin invaders. When Strongyloides larvae land on skin, they crawl briefly, then push their heads down into the surface, apparently sampling for soft spots or openings like hair follicles. Once they find a promising entry point, they puncture through head-first and burrow inward. If the spot is too tough, they pull back, reposition, and try again elsewhere.
Research using fluorescent imaging has captured this behavior in real time: the worms anchor their heads inside the skin while the rest of the body writhes freely on the surface, then gradually disappear below.
Once through the outer skin layer, these worms move fast. Strongyloides larvae can migrate through skin tissue at speeds up to 5 to 15 centimeters per hour, a rate that lets them reach blood vessels or lymph channels quickly and begin their journey to the lungs and gut.
Recent work has revealed that dopamine signaling within the worms themselves drives their decision to invade, suggesting the process is not random fumbling but a coordinated behavioral program triggered by chemical cues from host skin.
Blood Loss and Nutritional Theft
Once established in the gut, certain parasites feed directly on blood. Hookworms attach to the lining of the small intestine and chew into blood vessels, causing chronic bleeding that the host often does not notice until anemia sets in. Radioisotope studies have shown that people with heavy hookworm infections can lose up to about a quarter of a liter of blood per day, along with as much as 29 milligrams of iron, leading directly to iron-deficiency anemia.1The American Journal of Clinical Nutrition. Parasitism and Anemia Other blood-feeding parasites that cause similar iron-deficiency anemia include whipworms and the several species of Schistosoma that cause schistosomiasis.
The anemia from hookworm infection is compounded by a second problem. Research has found that people with severe iron-deficiency anemia from hookworms also experience excess destruction of red blood cells in the bloodstream, beyond what can be explained by intestinal blood loss alone. This excess hemolysis persists even after most of the worms are removed and only resolves once the anemia itself is treated with iron supplementation.2Blood. Excess Hemolysis in Subjects with Severe Iron Deficiency Anemia Associated and Nonassociated with Hookworm Infection In other words, the parasite sets off a cascade of problems that outlasts its physical presence.
Hookworms are not the only parasites that sabotage nutrition. Giardia lamblia, the protozoan that causes giardiasis, produces surface proteins that bind zinc and other metals in the colon, causing zinc malabsorption.3PubMed Central. The Impact of Parasitic Infestation on Nutritional Status and Micronutrients among Children Since many intestinal parasites also require iron for their own growth and reproduction, they effectively compete with the host for a nutrient the host can ill afford to lose. In children, this combination of blood loss, malabsorption, and nutrient competition can stunt growth and impair cognitive development.
Cysts in the Liver, Lungs, and Brain
Some parasites do not stay in the gut at all. The larval stages of certain tapeworms migrate to deep organs and form fluid-filled cysts that can grow for years before causing symptoms. The pork tapeworm (Taenia solium) can produce cysts, called cysticerci, in the brain, a condition known as neurocysticercosis that is one of the leading causes of acquired epilepsy worldwide. Echinococcus granulosus and Echinococcus multilocularis form cysts predominantly in the liver and lungs, but the brain and other organs are also at risk.4PubMed. Epidemiology, clinical manifestations and diagnosis of zoonotic cestode infections: an update
Echinococcus cysts grow slowly, sometimes over a decade, and can reach the size of a grapefruit before they cause enough pressure on surrounding tissue to produce symptoms. The alveolar form, caused by E. multilocularis, behaves almost like a slow-growing tumor, infiltrating the liver in a way that can be fatal if not treated. Surgical removal of these cysts is delicate because rupture can spill larval material into the abdominal cavity and trigger life-threatening allergic shock or seed new cysts elsewhere.
Hijacking the Immune System
Perhaps the most sophisticated thing parasites do is manipulate the body’s immune response so they can survive inside it for years. Your immune system is built to detect and destroy foreign invaders, and parasites have evolved an arms race of countermeasures.
One major strategy involves regulatory T cells, a subset of immune cells whose job is to tamp down immune activity and prevent autoimmune damage. Helminths (parasitic worms) actively induce these cells, either by secreting molecules that mimic the body’s own immune-calming signals or by interacting with other immune cells like dendritic cells that then generate regulatory T cells on the parasite’s behalf.5PubMed Central. Regulatory T-cells in helminth infection: induction, function and therapeutic potential The result is a host immune system that is partially switched to a tolerant mode, allowing the worms to persist. Parasites also release proteins, metabolites, and tiny membrane-bound packages called extracellular vesicles that directly activate signaling pathways inside host cells, further dampening the immune attack.6PubMed Central. Helminth parasites and immune regulation
Another evasion tactic, used especially by protozoan blood parasites like trypanosomes, is antigenic variation. The parasite constantly changes the molecular coat on its surface so that by the time your immune system produces antibodies against one version, the parasite has already switched to a different one. This cat-and-mouse cycle can continue indefinitely, making it extremely difficult for the body to clear the infection without treatment.
What Parasites Do to the Brain
Some of the most unsettling effects of parasitic infection involve the central nervous system. Toxoplasma gondii, the protozoan best known for its association with cats, forms cysts in brain tissue during chronic infection. Experimental studies have found that these cysts alter neurotransmitter levels: dopamine rises significantly while serotonin drops.7PubMed. The effect of chronic experimental toxoplasmosis on some brain neurotransmitters level and behavior changes The behavioral consequences in rodents are well documented, including reduced fear of predators. In humans, the picture is murkier, but chronic Toxoplasma infection has been linked in observational studies to subtle changes in risk-taking behavior and to associations (not proven causal) with psychiatric conditions. What is clear is that the parasite globally alters neurological signaling pathways during infection.8PubMed Central. Neurophysiological Changes Induced by Chronic Toxoplasma gondii Infection
African sleeping sickness, caused by the trypanosome Trypanosoma brucei, offers an even more dramatic example. The disease gets its name from the profound sleep disruption it causes. Research in mouse models has shown that the parasite shortens the body’s internal circadian clock, with the effect appearing within days of infection, before parasites are even detectable in the brain.9Nature Communications. Sleeping sickness is a circadian disorder This suggests the disruption begins through peripheral signals, possibly a hormone or metabolite released by the parasite, that feeds back onto the brain’s master clock. In humans, the result is excessive daytime sleepiness, fragmented nighttime sleep, and neuropsychiatric symptoms linked to inflammatory cytokines, altered neurotransmitter signaling, and disruption of clock-gene activity.10PubMed Central. Sleep-Wake Dysregulation in Human African Trypanosomiasis: From Neuroinvasion to Neuronal Dysfunction
More broadly, the neuroinflammation triggered by various parasites, both protozoan and helminth, has been associated with seizures, cognitive impairment, mood and anxiety symptoms, and in some cases psychosis-like states.11PubMed Central. From parasite-induced immune activation to neuroinflammation and behavioral dysfunction: convergent mechanisms across protozoa and helminths: a review Plasmodium falciparum, the malaria parasite, can invade or indirectly influence the central nervous system through neuroinflammation, sometimes producing significant behavioral or cognitive changes, especially in children who survive cerebral malaria.12PubMed. Parasite infections: how inflammation alters brain function
Chronic Inflammation and Organ Scarring
Many of the worst long-term consequences of parasitic infection come not from the parasite itself but from the body’s inflammatory response to it. Schistosomiasis provides the textbook example. The adult worms live in blood vessels around the intestine or bladder, and the females lay eggs that become trapped in the liver. The immune system walls off each egg in a ball of inflammatory cells called a granuloma. Over years, the accumulation of granulomas triggers fibrosis, essentially scarring the liver tissue and obstructing blood flow through it.13PubMed. Genistein improves schistosomiasis liver granuloma and fibrosis via dampening NF-kB signaling in mice Research has identified the soluble antigens released by the eggs as the main drivers of this process, activating inflammatory pathways that lead progressively to liver damage.14PubMed Central. Hepatic schistosomiasis as a determining factor in the development of hepatic granulomas and liver fibrosis: a review of the current literature
Advanced schistosomal liver fibrosis can cause portal hypertension, enlarged spleen, fluid accumulation in the abdomen, and life-threatening bleeding from swollen veins in the esophagus. These are the consequences not of a parasite eating your liver but of your own immune system trying, unsuccessfully, to destroy parasite eggs it cannot remove.
Damage to the Heart
The heart is not spared. Chagas disease, caused by the protozoan Trypanosoma cruzi and transmitted by “kissing bugs” in the Americas, is considered the most important parasitic infection of the heart globally. The parasite can directly invade heart muscle cells, and the chronic immune response to the infection can produce myocarditis (inflammation of the heart muscle), pericarditis (inflammation of the sac surrounding the heart), or a condition involving the entire heart wall. Over decades, Chagas disease can lead to dilated cardiomyopathy, heart failure, dangerous arrhythmias, and sudden cardiac death.15PubMed Central. Cardiac involvement with parasitic infections Because populations from endemic areas have migrated worldwide, Chagas cardiomyopathy now appears in cardiology clinics far from where the disease is traditionally found. Other parasites, including Schistosoma species and Echinococcus, can also affect the heart, producing pulmonary hypertension or cardiac cysts respectively.
Links to Cancer
A few parasitic infections are recognized carcinogens. The liver flukes Opisthorchis viverrini and Clonorchis sinensis, acquired from eating raw freshwater fish in parts of Southeast Asia and East Asia, cause chronic inflammation of the bile ducts that can progress to cholangiocarcinoma, a bile duct cancer. Schistosoma haematobium, which deposits its eggs in the bladder wall, is strongly linked to squamous cell carcinoma of the bladder. These helminth diseases are classified as highly carcinogenic.16PubMed Central. Parasite Infection, Carcinogenesis and Human Malignancy
Malaria itself does not directly cause cancer, but it is strongly associated with endemic Burkitt lymphoma in regions where malaria transmission is intense and year-round. The relationship appears to involve chronic immune stimulation and co-infection with Epstein-Barr virus, creating conditions that favor the lymphoma’s development. Meanwhile, Trypanosoma cruzi has a more ambiguous relationship with cancer: some evidence suggests carcinogenic properties, while other findings point to anticancer effects, a duality that researchers are still untangling.
Reshaping the Gut Microbiome
Parasites that live in the intestine do not exist in isolation. They share space with trillions of bacteria that make up the gut microbiome, and mounting evidence shows they reshape that microbial community. Across both clinical studies in infected people and experimental models, intestinal parasites have been shown to alter microbial composition and diversity.17PubMed Central. Interaction between Intestinal Parasites and the Gut Microbiota: Implications for the Intestinal Immune Response and Host Defence
A systematic review and meta-analysis found that intestinal helminth parasites tend to increase the diversity of gut bacteria and promote a reorganization of the microbial community structure. The effect was strongest for helminths residing in the large intestine, like pinworms and whipworms, suggesting the impact depends on which part of the gut the parasite occupies.18PubMed Central. Helminth-Induced Human Gastrointestinal Dysbiosis: a Systematic Review and Meta-Analysis Reveals Insights into Altered Taxon Diversity and Microbial Gradient Collapse Whether these microbiome changes help or harm the host is context-dependent. In some cases, the shifts may contribute to the immune dampening that helps the parasite survive; in others, they could have knock-on effects on digestion, nutrient absorption, or susceptibility to bacterial infections.
Skin Parasites and Secondary Infections
Not all parasites operate from the inside. Ectoparasites like the scabies mite (Sarcoptes scabiei) burrow into the outermost layer of skin and provoke intense itching. The itch is not just uncomfortable. It disrupts the skin barrier, and the scratching that follows opens the door to secondary bacterial infections. Streptococcal and staphylococcal superinfections of scabies lesions are common, and in some populations these skin infections can lead to serious downstream problems like kidney disease.19PubMed Central. Itch in Scabies-What Do We Know? Crusted (Norwegian) scabies, a severe form in which thousands of mites infest the skin, is particularly dangerous in people with compromised immune systems and is highly contagious.
Hormonal and Metabolic Disruption
Parasites can interfere with the body’s hormonal balance in ways that are still being mapped. Research indicates that parasites disrupt the synthesis, secretion, metabolism, and action of the host’s own hormones, either through the exaggerated immune response they provoke or by directly affecting endocrine tissues.20PubMed. Environmental parasitology and its impact on the host nueroimmunoendocrine network
Some of these metabolic effects are surprisingly nuanced. Parasitic nematodes can alter gut hormones, change the function of the intestinal lining, and shift the number and behavior of immune cells in metabolic tissues like fat. Through immune pathways that activate genes regulating glucose and fat metabolism, nematode infections have been shown to improve insulin sensitivity in animal models.21PubMed Central. Parasites, nutrition, immune responses and biology of metabolic tissues This paradoxical finding, that infection with a harmful organism could improve a metabolic marker, ties into a broader and increasingly active area of research.
When Parasites Might Protect Against Other Diseases
Despite everything described above, there is a genuine scientific case that some helminth infections may protect against autoimmune and inflammatory diseases. The idea, often grouped under the “hygiene hypothesis,” rests on the observation that the immune-dampening effects helminths use for their own survival can also suppress the overactive immune responses behind conditions like inflammatory bowel disease, multiple sclerosis, rheumatoid arthritis, and type 1 diabetes.22PubMed Central. Unraveling the Hygiene Hypothesis of helminthes and autoimmunity: origins, pathophysiology, and clinical applications
Several phylogenetically distinct helminth species, from tapeworms to blood flukes to roundworms, have been shown in animal models to reduce gut inflammation, largely through the release of the immune-calming molecule IL-10.23PubMed Central. The Hygiene Hypothesis and Its Inconvenient Truths about Helminth Infections And evidence suggests helminths can dampen pathology even in established inflammatory diseases, meaning the protection is not limited to people who grew up exposed to worms.24PubMed Central. Helminths in the hygiene hypothesis: sooner or later?
Clinical trials using helminth-derived products or controlled low-dose infections have been attempted for several autoimmune conditions, though results so far have been mixed. The challenge is separating the beneficial immune modulation from the genuine tissue damage, nutritional theft, and cancer risk that come with actual parasitic infection. Researchers are increasingly focused on isolating the specific molecules parasites use to quiet the immune system, in hopes of developing drugs that deliver the benefit without the worm.
How Malaria Shaped the Human Genome
The relationship between humans and parasites is old enough that it has left marks in our DNA. Malaria, one of the deadliest parasites in human history, has exerted some of the strongest selective pressures ever measured on the human genome, particularly on red blood cells, its primary target.25PubMed Central. The ultimate tradeoff: how red cell adaptations to malaria alter the host response during critical illness Genetic variants like sickle cell trait, thalassemia, and glucose-6-phosphate dehydrogenase deficiency all reached high frequencies in malaria-endemic populations because they confer partial resistance to the parasite. Each comes with its own health costs, especially when inherited from both parents, but the survival advantage against malaria was powerful enough to maintain these variants across hundreds of generations. That a single parasitic organism could reshape the blood chemistry of entire populations speaks to the depth and duration of the human-parasite relationship.