Soil-transmitted helminths, a group of parasitic worms that live in the human intestine, are the most common parasitic infections worldwide. An estimated 1.5 billion people are currently infected, roughly 24% of the global population. The single most prevalent species is the large roundworm (Ascaris lumbricoides), which infects nearly 294 million people. But the answer shifts depending on where you live and which type of parasite you’re counting.
Soil-Transmitted Helminths by the Numbers
Three species of parasitic worm account for the bulk of human infections on Earth. Roundworm leads with an estimated 293.8 million cases as of the most recent WHO data from 2021. Whipworm follows at 255.9 million, and hookworm adds another 112.8 million. Together, these three worms require preventive treatment for over 876 million children across 86 countries.
All three spread through soil contaminated with human feces containing parasite eggs or larvae, which is why they cluster in regions with limited sanitation. Roundworm and whipworm eggs mature in soil and enter the body when someone eats unwashed vegetables, drinks contaminated water, or touches contaminated soil and then their mouth. Hookworm works differently: its larvae hatch in soil and actively burrow through the skin, most commonly through bare feet. One hookworm species can also spread through contaminated food.
The good news is that these numbers are declining. Hookworm infections dropped by nearly 30% between 2015 and 2021, and roundworm infections fell by 21%. Whipworm has been more stubborn, declining only 5.6% in that same period. The WHO defines elimination as a public health problem when fewer than 2% of people in an area carry moderate or heavy infections of any of these species.
Common Parasites in the United States
In higher-income countries, the picture looks very different. Soil-transmitted worms are rare, but other parasites are surprisingly widespread. Toxoplasmosis tops the list in the U.S., with more than 60 million Americans chronically infected with the parasite Toxoplasma gondii. Most carriers never know it. The parasite spreads through undercooked meat containing tissue cysts, or through contact with contaminated soil, water, or unwashed produce. Cat litter is a well-known source of the parasite’s eggs.
Trichomoniasis, a sexually transmitted infection caused by a single-celled parasite, is another major one. The CDC estimates over two million infections occurred in the U.S. in 2018, making it the most common curable STI in the country. It often causes no symptoms, particularly in men, so many cases go undiagnosed and untreated.
The CDC has flagged five parasitic infections as neglected priorities within the U.S.: Chagas disease (over 300,000 infections), cysticercosis (at least 1,000 hospitalizations per year), toxocariasis (14% of the population has been exposed), toxoplasmosis, and trichomoniasis. These get far less attention than their prevalence warrants, partly because they disproportionately affect lower-income communities.
Pinworm in Children
For parents searching this topic, pinworm is worth knowing about. It’s the most common worm infection in temperate, developed countries, and up to 10% of children in these regions carry it at any given time. Pinworm spreads easily in schools and households because its tiny eggs survive on surfaces and are ingested through hand-to-mouth contact. Younger children are more likely to be infected: studies show prevalence around 3.8% in first graders, dropping to 1% by sixth grade. A single dose of standard deworming medication clears the infection in about 96% of cases.
Malaria: The Deadliest Parasite
Malaria isn’t the most common parasitic infection by total number of people carrying it at once, but it’s by far the most dangerous. In 2023, there were an estimated 263 million malaria cases and 597,000 deaths worldwide. The parasite spreads through mosquito bites rather than contaminated soil or food, which makes prevention a completely different challenge. Sub-Saharan Africa bears the overwhelming majority of both cases and deaths.
Where soil-transmitted worms cause chronic illness over years, malaria can kill within days if untreated. This distinction matters: “most common” and “most deadly” are very different metrics in parasitology.
How Parasitic Infections Feel
Many parasitic infections produce no symptoms at all, especially at low levels. When symptoms do appear, they tend to overlap regardless of which parasite is involved. The most common signs include fever, fatigue, muscle aches, nausea, vomiting, and diarrhea. Intestinal worms specifically can cause abdominal pain, bloating, and visible worms in stool.
Heavy roundworm infections can block the intestines, particularly in children. Hookworm feeds on blood and causes iron-deficiency anemia over time, leading to fatigue and developmental delays in kids. Whipworm in large numbers causes chronic diarrhea and rectal prolapse. These complications are rare in light infections but common where reinfection is constant and treatment is limited.
Toxoplasmosis, despite infecting millions, almost never causes illness in healthy adults. The immune system keeps the parasite dormant. It becomes dangerous primarily during pregnancy, when it can cross to the fetus, or in people with weakened immune systems. Trichomoniasis, when symptomatic, causes genital irritation, burning during urination, and unusual discharge.
How These Infections Spread
Parasites reach humans through three main routes. The first is the fecal-oral pathway: eggs or cysts shed in an infected person’s stool contaminate soil, water, or food, and another person swallows them. This is how roundworm, whipworm, and many protozoan parasites like Toxoplasma spread. The second is direct skin penetration, which is how hookworm larvae enter through bare feet. The third is vector-borne transmission, where an insect carries the parasite from one host to another, as mosquitoes do with malaria.
Clean water, sanitation, shoes, and proper food handling eliminate most of the risk. This is why the global burden of parasitic infection maps almost perfectly onto poverty. The WHO’s current roadmap for neglected tropical diseases, which covers the period through 2030, focuses on integrated approaches that address sanitation infrastructure alongside direct treatment, rather than tackling each parasite species in isolation.