How Common Are Parasites in Humans in the United States?

Parasitic infections in the United States are far more common than most people assume. Tens of millions of Americans carry antibodies to at least one parasite, and several million have active infections at any given time. The CDC classifies Chagas disease, cysticercosis, and toxoplasmosis as neglected parasitic infections precisely because they affect millions of people domestically yet receive relatively little attention or funding.¹ Beyond those three, a range of other parasites circulate through water, food, soil, pets, ticks, and sexual contact, collectively making the U.S. parasite picture more complicated than the “developed country, no parasites” story most of us grew up hearing.

Toxoplasmosis Is the Biggest Number Most People Don’t Know About

If you had to pick the single most prevalent parasitic infection in the country, it would be Toxoplasma gondii. A nationally representative survey covering 2009 through 2010 found that about 13% of people aged six and older had antibodies against the parasite, meaning they had been infected at some point.² That translates to roughly one in eight Americans. The good news is that this number has been dropping steadily: among U.S.-born people aged 12 to 49, exposure fell from about 14% in the late 1980s–early 1990s to about 7% by 2009–2010.²

Most people who carry the parasite never notice symptoms; the organism typically stays dormant in tissue cysts for life. The real danger is for pregnant women, who can transmit the infection to the fetus, and for people with weakened immune systems, in whom dormant cysts can reactivate and cause severe brain inflammation. The decline in seroprevalence likely reflects improvements in food handling and meat preparation over the past several decades, but at roughly one in eight people, the baseline remains surprisingly high.

A Sexually Transmitted Parasite Affecting Millions

Trichomonas vaginalis rarely makes headlines the way chlamydia or gonorrhea does, but it is one of the most common sexually transmitted infections in the country. A 2018 estimate put the number of prevalent infections among 15- to 59-year-olds at about 2.6 million, with women carrying the vast majority of infections (roughly 2.1 million) compared with men (about 470,000).³ National surveys from 2013–2016 found an overall prevalence of about 1.3% in urine samples, but that headline figure masks a stark disparity: prevalence among non-Hispanic Black women was roughly 10%, almost twenty times higher than in other groups.⁴

Income plays a role, too. People living below the federal poverty level had over six times the odds of infection compared with those at higher income levels, after controlling for other factors.⁵ Active smoking was also strongly linked to infection, with smokers having more than eight times the adjusted odds.⁵ Because the infection often causes no symptoms in men and can be mild or intermittent in women, many cases go undiagnosed and untreated, which fuels ongoing transmission.

Waterborne Parasites and the Swimming-Pool Problem

Giardia and Cryptosporidium are the two parasites most likely to make you sick through water in the United States. Between 1971 and 2011, 242 documented giardiasis outbreaks affected about 41,000 people. Most of those outbreaks (about three-quarters) were waterborne, with contaminated drinking water being the leading route, followed by recreational water like pools and splash pads.⁶ Problems with water treatment declined over the decades, but outbreaks tied to untreated groundwater have persisted, and areas that lack public water supply systems tend to see more giardiasis cases. One national analysis found that each 25% increase in the share of a county relying on private wells was associated with a 25% increase in giardiasis cases.⁷

Cryptosporidium is a trickier adversary because its oocysts can survive standard chlorine levels in swimming pools. Recreational-water outbreaks driven by Cryptosporidium have been rising. A single focal outbreak in a treated pool can seed secondary transmission across an entire community when swimmers visit multiple venues.⁸ This is why public-health agencies emphasize that people with diarrhea should stay out of swimming pools for at least two weeks after symptoms resolve, a recommendation many swimmers either don’t know about or disregard.

Parasites From Soil and Pets

Toxocara, the roundworm that dogs and cats shed in their feces, has left a bigger mark on the American population than you might expect. A nationally representative survey found that about 5% of people tested positive for Toxocara antibodies, indicating past exposure.⁹ That is roughly one in twenty Americans, with higher rates among men, people with lower incomes, and those with less formal education. In most people, the larvae wander harmlessly through tissue and eventually die, but in unlucky cases they can lodge in the eye (causing vision loss) or migrate to the brain. Children are at particular risk because they are more likely to play in soil contaminated with pet feces and to put dirty hands in their mouths.

Pinworm is another soil- and household-associated parasite that remains common, especially in children. Most infections cause nothing worse than nighttime itching around the anus, though severe cases can disrupt sleep and lead to skin infections from scratching.¹⁰ Pinworm spreads readily in households and daycare settings because its eggs are tiny, lightweight, and can survive on surfaces for weeks. Reliable national prevalence data are scarce, partly because diagnosis relies on the “tape test” rather than blood work, and many cases are managed at home without ever entering surveillance data.

Hookworm in Alabama and the Sanitation Gap

Hookworm was once the defining parasitic disease of the American South. In the early 1900s, the Rockefeller Sanitary Commission surveyed eleven southern states and found that about 40% of school-aged children were infected.¹¹ Massive treatment and education campaigns drove rates down dramatically over the following decades, and hookworm eventually disappeared from mainstream medical attention.

Then, in 2017, researchers went looking in Lowndes County, Alabama, a predominantly Black rural county where many homes still discharge raw sewage through “straight pipes” directly onto the ground. Among 55 stool samples collected, about 35% tested positive for the hookworm Necator americanus, and another 7% were positive for Strongyloides stercoralis, a related soil-transmitted worm.¹² Over 40% of the households in the study reported direct exposure to raw sewage near their homes. A follow-up study of 39 residences in the same county found that over 80% relied on on-site sanitation systems, most of which were failing, and about 31% of soil samples contained environmental and zoonotic parasites.¹³

The Lowndes County findings do not mean hookworm is widespread across the country. But they illustrate that where sanitation infrastructure has failed, parasites that most Americans consider eradicated can persist. Kentucky, for instance, has a uniquely elevated history of endemic strongyloidiasis, and internet search data suggest ongoing awareness of the problem in that state.¹⁴ These are pockets, not a national crisis, but they challenge the assumption that adequate sanitation is universal across the U.S.

Tick-Borne and Insect-Borne Parasites

Babesiosis, caused by the tick-transmitted parasite Babesia microti, is spreading geographically alongside Lyme disease in the northeastern United States. A serosurvey of blood donors found a 1.1% antibody prevalence in Connecticut and 1.4% on the offshore islands of Massachusetts, with positive donors turning up in every month of the year, not just during peak tick season.¹⁵ The geographic range of babesiosis has expanded, though its spread has lagged somewhat behind that of Lyme disease.¹⁶

Chagas disease, caused by the protozoan Trypanosoma cruzi and transmitted by “kissing bugs” (triatomine insects), is another parasite many Americans think of as exclusively tropical. Most domestic cases are in immigrants who were infected in Latin America, but locally acquired transmission does occur. A case report from rural Louisiana documented an autochthonous infection; when researchers collected kissing bugs from the patient’s home, 56% of them carried the parasite.¹⁷ All blood donors in the United States are now screened at least once for T. cruzi antibodies.¹⁸

Foodborne Parasites and the Fresh-Produce Connection

Cyclospora cayetanensis has become a recurring summer headache for food-safety investigators. This protozoan causes prolonged, relapsing watery diarrhea and is overwhelmingly linked to contaminated fresh produce like berries, leafy greens, and herbs.¹⁹ Recent multistate outbreaks have highlighted how hard it is to trace the contamination source, because produce has a short shelf life, supply chains cross many borders, and leftovers are rarely available for testing by the time an outbreak is recognized. Outbreaks linked to domestically grown produce have also been documented, pushing researchers to investigate the parasite’s presence in U.S. agricultural environments themselves.²⁰

Neurocysticercosis, another foodborne parasite (acquired by ingesting eggs of the pork tapeworm Taenia solium), is the leading parasitic cause of seizures in the country. A study of emergency-department patients who had seizures and underwent brain imaging found that about 2% met the case definition for neurocysticercosis.²¹ Cases are concentrated among immigrants from endemic regions in Latin America, but anyone exposed to the eggs through fecal-oral contamination can develop the disease. At one New York hospital, about 41% of neurocysticercosis patients presented with seizures in the emergency department.²²

Why the Numbers Are Almost Certainly Undercounts

Several forces conspire to keep reported parasite numbers lower than the true burden. Many parasitic infections cause no symptoms or only vague digestive complaints that patients and doctors alike chalk up to something else. Pinworm, Trichomonas, and Blastocystis are all examples of infections that can persist quietly for months or years. In a small U.S. study of 139 individuals, about 7% tested positive for Blastocystis, a gut protozoan whose clinical significance remains debated, and all positive cases were in different family units, suggesting that much of the exposure happens sporadically rather than in household clusters.²³

Physician awareness also plays a role. Many U.S. medical training programs devote minimal time to parasitology because parasitic disease is widely perceived as a “tropical” problem. A family physician in the Midwest or Northeast who sees a patient with chronic diarrhea is likely to order stool cultures for bacteria long before thinking about Giardia or Cyclospora. Immigrants and refugees, who may carry infections acquired abroad, face additional diagnostic barriers including language, insurance gaps, and the stigma that can accompany parasite diagnoses.²⁴

Protecting the Blood Supply

One domain where the U.S. has taken parasites seriously is blood-bank safety. Babesia microti is the most frequently reported transfusion-transmitted parasitic infection in the country, and the consequences for immunocompromised recipients can be severe. A large study in Connecticut and Massachusetts found zero cases of transfusion-transmitted babesiosis among screened donations, compared with 14 cases among unscreened donations from the same period.²⁵ That screening program has since expanded. Chagas screening of the blood supply, as noted earlier, is now universal for all U.S. donors, with military donors screened at every donation rather than just once.¹⁸

These screening protocols exist precisely because the parasites are circulating widely enough to pose a real transfusion risk. The seroprevalence figures are the quiet evidence: when roughly one in a hundred blood donors in endemic northeastern counties carries Babesia antibodies, not screening would guarantee that contaminated units would reach patients on a regular basis.

Where Poverty and Parasites Intersect

If there is a single theme that ties together the disparate parasites described in this article, it is that infection burden in the U.S. maps closely onto poverty and infrastructure failure. Hookworm in Alabama tracks with failing septic systems and straight-pipe sewage. Trichomonas disparities align with income and racial health inequities. Toxocara exposure is highest among people with lower incomes and less education. Even Giardia outbreaks cluster in areas without centralized water treatment.

This pattern matters because it means the national averages can be misleading. A seroprevalence of 5% for Toxocara sounds modest until you consider that the rate in some communities may be far higher. The Lowndes County hookworm data, where over a third of tested residents were positive, are a dramatic example of how a national average can obscure what is happening in specific places. Parasitic disease in the U.S. is not evenly distributed; it is concentrated in communities that already bear a disproportionate burden of other health problems, making it easy for the broader public and policymakers to overlook.

The Rockefeller hookworm campaign a century ago showed that targeted investment in sanitation and treatment can wipe out a parasite remarkably quickly.¹¹ The fact that hookworm turned up again in essentially the same region, in communities where sanitation funding dried up or never arrived, is a reminder that the problem is less about the biology of the parasite than about the infrastructure surrounding the people it infects.