How Do You Get Parasites? Causes and Risk Factors

Parasites reach humans through a surprisingly wide range of routes: contaminated water, undercooked food, insect bites, bare skin touching soil, close contact with pets, person-to-person spread, and even transmission from a pregnant mother to her unborn child. No single “cause” explains parasitic infections because different organisms have evolved radically different strategies for getting inside a human host. Understanding those routes is the first step toward knowing which risks actually apply to your life and which ones you can reduce.

Contaminated Water

Drinking water is one of the most common vehicles for parasitic infection worldwide. Two microscopic protozoa, Giardia and Cryptosporidium, are responsible for the majority of waterborne parasitic disease outbreaks globally, largely because their cyst forms are tough enough to survive standard water treatment that kills most bacteria.¹ Both organisms cause diarrheal illness that can last weeks if untreated, and Cryptosporidium in particular is notoriously resistant to chlorine at the concentrations used in municipal water systems.

This is not exclusively a problem of low-income countries. Outbreaks linked to municipal water supplies have been documented across Europe, North America, and the Middle East. The risk goes up wherever water treatment is outdated, where agricultural runoff contaminates reservoirs, or where you are drinking from untreated surface sources like streams and lakes while hiking or traveling. Boiling water or using filters rated for protozoan cysts effectively eliminates the threat, but many people in resource-limited settings lack reliable access to either.

Cooking water matters too, not just drinking water. A study in Jakarta found that people who did not treat their cooking water had roughly double the odds of intestinal parasitic infection compared to those who did, even in a community with generally improved water access.² That finding is a useful reminder: water you use to wash vegetables or cook partially heated dishes can carry the same parasites as water you drink straight from the tap.

Food, Especially Undercooked Meat

Three parasites in particular are associated with eating raw or undercooked pork: the roundworm that causes trichinosis, the pork tapeworm, and Toxoplasma gondii.³ Pork gets the most attention, but beef, lamb, wild game, and freshwater fish all carry their own parasite risks when consumed undercooked. Toxoplasma alone infects an estimated one-third of the world’s population, often through meat.

Raw or rare meat isn’t the only food concern. Unwashed produce can carry parasite eggs deposited by contaminated soil or irrigation water. A recent modeling study estimated that the per-portion risk of Toxocara infection from undercooked meat was about ten times higher than from unwashed spinach, though the spinach risk was not zero.⁴ The practical lesson: cooking meat to recommended internal temperatures eliminates most parasitic risk, and thoroughly washing leafy greens and root vegetables under running water handles the rest. Freezing meat at sufficiently low temperatures for several days also kills many parasitic larvae, which is why sushi-grade fish is typically frozen before sale.

Walking Barefoot and Soil Contact

Some parasites don’t need you to eat or drink anything. They come to you. Soil-dwelling hookworm larvae and Strongyloides larvae actively navigate toward hosts using sensory cues like body heat and skin odor, then burrow directly through the skin on contact.⁵ Walking barefoot on warm, moist soil contaminated with human feces is the classic route, and it remains a real risk in areas with poor sanitation. The larvae typically enter through the soles of the feet, migrate through the bloodstream to the lungs, get coughed up and swallowed, and then set up camp in the small intestine.

This route of infection was once widespread in the rural American South. A study in Lowndes County, Alabama, found that more than a third of stool samples tested positive for the hookworm Necator americanus, in a community where over 40% of households reported exposure to raw sewage near their homes.⁶ The finding was striking because hookworm had been considered essentially eradicated in the United States since mid-twentieth-century sanitation campaigns. Wearing shoes outdoors in areas with questionable sanitation is the simplest prevention.

Insect Bites and Other Vectors

Mosquitoes transmitting malaria are the best-known example of vector-borne parasitic disease, but they are far from the only insect involved. Triatomine bugs, sometimes called “kissing bugs,” transmit Trypanosoma cruzi, the parasite that causes Chagas disease. These insects feed on sleeping hosts at night and defecate near the bite wound; the parasite enters through the broken skin or mucous membranes when the person scratches the bite. Research on triatomines collected in Arizona and California found that a high proportion had fed on humans and that some carried T. cruzi, indicating real transmission potential in the United States.⁷

In a more unexpected finding, laboratory research has shown that common bed bugs can also serve as competent vectors for T. cruzi. Bed bugs acquired the parasite by feeding on infected mice and then transmitted it back to uninfected hosts, both during cohabitation and through contaminated feces placed on broken skin.⁸ This does not mean bed bug infestations routinely cause Chagas disease, but it raises the possibility in areas where the parasite is circulating. Sand flies transmit leishmaniasis, tsetse flies transmit sleeping sickness in sub-Saharan Africa, and blackflies transmit river blindness. In all these cases, the parasite has evolved a life stage specifically adapted to the insect, making the bug an essential part of the transmission cycle rather than a passive carrier.

Pets and Domestic Animals

Dogs and cats are beloved household members and also reliable sources of several parasitic infections. Dogs can transmit intestinal parasites, visceral larva migrans (caused by migrating Toxocara worm larvae), ringworm, and echinococcosis, among other conditions.⁹ The organisms typically reach people through contact with contaminated feces, soil where pets have defecated, or, less commonly, through direct licking of broken skin.

Pet dogs and cats excrete Toxocara eggs directly into the human environment without requiring any intermediate host or insect vector.¹⁰ Children playing in parks or sandboxes contaminated with dog feces are at particular risk of ingesting Toxocara eggs, which can then migrate through tissues and occasionally reach the eye, causing vision problems. Regular deworming of pets, prompt disposal of pet feces, and handwashing after handling animals are the main defenses. Cats carry the additional risk of being the definitive host for Toxoplasma gondii; cleaning a litter box without gloves and handwashing is a well-known route of exposure, which is why pregnant women are often advised to have someone else handle the litter box.

Person-to-Person Spread

Not all parasites require contaminated food, water, or an insect go-between. Pinworm is perhaps the most common parasitic infection in high-income countries, and it spreads almost entirely from person to person. The female worm migrates to the perianal area at night to deposit eggs, which end up on fingers, bedding, clothing, and household surfaces. The main risk factors are predictable: being a school-age child, nail-biting, poor hand hygiene, and close living quarters. Treatment works well, but eradication requires treating everyone in the household, because reinfection from a missed carrier is nearly guaranteed otherwise.¹¹

What many people don’t realize is that pinworm eggs can also become airborne. Shaking out contaminated bedding or clothing can launch eggs into the air, where they are inhaled or swallowed.¹² The eggs are microscopic and can remain viable on surfaces for two to three weeks at room temperature, which explains why outbreaks in daycares and schools are so common and so stubborn. Scabies mites and head lice, while not internal parasites, also spread through direct skin-to-skin contact or shared personal items, following the same general person-to-person logic.

Swimming in Lakes and Freshwater

Recreational swimming in freshwater lakes carries a specific parasitic risk that often catches people off guard: swimmer’s itch, a skin rash caused by larval forms of bird schistosomes released from infected snails.¹³ The larvae are meant to find a bird host, but when they accidentally penetrate human skin, they trigger an itchy allergic reaction. The parasites die in human skin and cannot complete their life cycle, but the rash can be miserable for days.

Research on lakes in the midwestern United States found that all episodes of swimmer’s itch occurred on days involving shallow-water use, with incidence highest in certain lake zones.¹⁴ Wading in shallow water near shoreline vegetation, where snails concentrate, is riskier than swimming in open deep water. Toweling off vigorously immediately after leaving the water can reduce penetration, because the larvae tend to burrow in as water evaporates from the skin. The risk varies enormously from lake to lake and even from one section of shoreline to another, depending on snail populations, bird populations, water temperature, and wind patterns.¹⁵

In tropical regions, a far more serious version of waterborne skin penetration exists: schistosomiasis, caused by schistosome species that can complete their life cycle in humans. Contact with freshwater in endemic areas of Africa, Southeast Asia, and parts of South America is the primary risk.

Mother-to-Child Transmission

A handful of parasites can cross the placenta and infect a developing fetus. The three most significant are Toxoplasma gondii, Trypanosoma cruzi (the Chagas parasite), and Plasmodium species (malaria).¹⁶ Congenital toxoplasmosis is the one most commonly discussed in prenatal care. If a woman acquires her first Toxoplasma infection during pregnancy, the parasite can cross to the fetus. The risk of transmission rises dramatically as pregnancy progresses: it is low in the first trimester but can reach 90% in the final days before delivery.¹⁷ Paradoxically, infections acquired earlier in pregnancy tend to cause more severe damage to the fetus despite being less likely to transmit.

Congenital Chagas disease affects an estimated several thousand newborns annually in Latin America. Screening pregnant women and treating infected infants early improves outcomes, but the condition is frequently missed, especially when families emigrate to non-endemic countries where clinicians don’t think to test for it. Congenital malaria, while less common than mosquito-borne malaria, occurs when infected maternal blood cells cross the placenta during delivery.

Medical Settings and Blood Products

Transmission of parasites through blood transfusions and organ transplants is rare but documented. Malaria, Chagas disease, babesiosis, and toxoplasmosis have all been transmitted through donated blood or organs.¹⁸ Blood banks in endemic areas screen for some of these parasites, and in the United States, donor questionnaires exclude people with recent travel to malaria-endemic zones. Babesiosis, transmitted by ticks in the northeastern United States, has become the most common transfusion-transmitted parasitic infection in North America because infected donors may be asymptomatic and unaware of their status. Testing for babesiosis in donated blood has been expanding in recent years.

Sanitation Infrastructure as the Underlying Risk Factor

Many of the routes described above share a common upstream cause: inadequate sanitation. When human waste is not separated from living environments, parasites that depend on the fecal-oral cycle or on larvae developing in contaminated soil thrive. The impact of improving sanitation is measurable. A citywide sewerage program in northeast Brazil cut the prevalence of several intestinal parasites roughly in half, with reductions of about 45% for roundworm, 65% for whipworm, and nearly 60% for Giardia.¹⁹ The analysis attributed a substantial portion of the decline specifically to sewer connections rather than to other concurrent improvements.

A large review of rigorous studies from multiple countries found median reductions of about 29% for roundworm infections, 26% for diarrheal disease, and over 75% for schistosomiasis and dracunculiasis when water supply and sanitation improved.²⁰ Brazilian national data spanning decades further showed that access to sanitary sewer systems had a protective effect against hookworm infection at the population level.²¹

What’s striking is that hygiene education alone, without infrastructure changes, often produces disappointing results. An intervention study in rural Ethiopia that provided water, sanitation, and hygiene education to households found significant improvements in household practices but only a small, statistically insignificant drop in actual parasite prevalence.²² Knowledge alone does not substitute for the physical infrastructure that keeps human waste out of water and soil. People can know exactly what to do and still get infected if the pipes and latrines aren’t there.

Your Immune System and Gut Bacteria

Two people can share the same contaminated water source and have very different outcomes. Part of the explanation is immune status: malnourished individuals, people with HIV, organ transplant recipients on immunosuppressive drugs, and young children with immature immune systems are all more susceptible to parasitic infection and more likely to develop severe disease from it.

The gut microbiome also plays a role that researchers are still working to understand. The bacterial populations in your intestines interact with parasites in both directions. A healthy gut microbiome can protect against parasitic colonization, while disruption of those bacterial communities can make infection more likely and more severe.²³ Conversely, established parasites can reshape the gut microbial community in ways that favor their own survival.²⁴ The specifics are still being mapped out, but the practical implication is that overall gut and immune health is part of your defense even when exposure cannot be avoided entirely.

Climate, Geography, and Shifting Risk Zones

Where you live determines which parasites you’re most likely to encounter. Tropical and subtropical climates support the widest range of parasitic threats because warm, moist conditions favor the survival of larvae in soil, the breeding of insect vectors, and the development of parasite stages in snails and other intermediate hosts. Hookworm, schistosomiasis, and malaria are all far more prevalent in equatorial regions for exactly these reasons.

Climate change is shifting these boundaries. Both direct effects, like rising temperatures expanding the geographic range of vector insects, and indirect effects, like altered rainfall patterns creating new breeding habitats, are expected to change where parasitic diseases appear.²⁵ Diseases once confined to the tropics are being documented at higher latitudes and altitudes. Leishmaniasis, for example, has been creeping northward in southern Europe. For travelers, this means that risk assessments based on outdated maps may underestimate current exposure in regions that are newly hospitable to parasites and their vectors.

How Parasites Manipulate Their Own Transmission

One of the more unsettling aspects of parasitology is that some parasites do not passively wait for transmission opportunities. They actively alter the behavior of their current host to increase the chances of reaching the next one.²⁶ The classic example is Toxoplasma gondii, which makes infected rodents less fearful of cats, effectively delivering itself to the predator it needs as a definitive host. Parasitic hairworms drive their cricket hosts to leap into water, where the worm can emerge and reproduce. Lancet liver flukes cause infected ants to climb to the tips of grass blades at dusk, exactly where a grazing cow or sheep is most likely to eat them.

These are not random side effects of infection. The behavioral changes are often precise, targeted, and repeatable across infected individuals. They represent evolutionary adaptations that have been refined over millions of years of host-parasite coevolution. Whether Toxoplasma meaningfully alters human behavior remains debated, though associations between Toxoplasma infection and subtle changes in risk-taking and reaction time have been reported. For the practical question of how you get parasites, the takeaway is that the encounter is sometimes not accidental on the parasite’s end, even if it feels that way on yours.

Parasites Through Deep History

Humans have carried parasites for as long as there have been humans, and likely longer. Archaeological evidence from preserved feces and mummified remains shows that intestinal parasites have been with hominids since at least the time of Homo erectus.²⁷ The shift from hunting and gathering to agriculture did not uniformly increase parasitic burden, but it changed the mix. In Europe, the domestication of animals brought new zoonotic parasites from livestock, while in the Americas, the first agricultural transition did not produce the same spike in animal-derived infections.²⁸ Urbanization later created ideal conditions for fecal-oral parasites by concentrating people and their waste, and it was only the development of modern sewerage and water treatment in the nineteenth and twentieth centuries that broke those cycles in industrialized regions. The parasites never disappeared, though. They retreated to wherever sanitation remains inadequate, which turns out to include pockets of every country on earth.