Parasites reach your stomach and intestines primarily through contaminated food, water, soil, or contact with infected animals. Most infections follow what’s called a fecal-oral route: parasite eggs or cysts shed in the feces of an infected person or animal eventually make their way into someone else’s mouth, usually on unwashed hands, in untreated water, or on contaminated food. Roughly 1.5 billion people worldwide, about 24% of the global population, carry at least one type of intestinal parasite.
Contaminated Water
Water is one of the most common vehicles for intestinal parasites, particularly microscopic single-celled organisms like Giardia and Cryptosporidium. These parasites form protective shells called cysts that are distributed worldwide in lakes, ponds, rivers, and streams. According to the EPA, virtually all surface waters contain Giardia, and levels tend to be higher near agricultural operations like cattle or dairy farming, or near wastewater discharge points. Cysts have also been found in wells contaminated by surface water or sewage.
In the United States, increased risk has been documented in communities that drink unfiltered surface water, people who use shallow well systems, hikers and campers who drink from streams, and anyone who accidentally swallows water while swimming in lakes or pools. The cysts are tougher than bacteria and viruses, meaning standard chlorine disinfection may not eliminate them unless the dose is high enough and the contact time long enough. Filtration is generally more reliable.
Undercooked or Raw Meat
Eating meat that hasn’t been cooked to a safe internal temperature is a direct route for several parasites. Pork is the classic source of a roundworm infection called trichinosis, though recent U.S. outbreaks have also been linked to wild boar, bear, and walrus meat. The parasites live inside the muscle tissue of the animal. When you eat that meat raw or undercooked, the larvae survive digestion, settle in your intestines, and begin reproducing.
Toxoplasma, another common parasite, can be present in virtually any type of meat. Raw or rare beef, lamb, and venison are frequent sources. Fish carries its own risks, particularly from parasitic worms that can embed in your stomach lining if the fish isn’t cooked thoroughly.
The CDC recommends these minimum internal temperatures to kill parasites:
- Whole cuts of meat (beef, pork, lamb): 145°F (63°C), then rest for three minutes before eating
- Ground meat: 160°F (71°C), no rest time needed
- All poultry: 165°F (74°C)
- Fish: 145°F, or until the flesh is opaque and flakes easily
Soil Contact
Soil-transmitted parasites are exactly what they sound like: worms whose eggs or larvae live in dirt contaminated with human feces. This is a massive global health problem, with over 1.5 billion people infected. The main culprits are roundworms, whipworms, and hookworms, and each has a slightly different entry method.
Roundworm and whipworm eggs are swallowed. This typically happens when contaminated soil gets on your hands and you touch your mouth without washing, or when you eat unwashed produce grown in contaminated ground. Children playing in dirt are especially vulnerable. Hookworms work differently. Their eggs hatch in the soil and release larvae that can burrow directly through your skin, usually the soles of your feet. Walking barefoot on contaminated ground is the primary risk.
Contact With Pets and Animals
Dogs and cats carry their own species of roundworms, and those parasites can infect humans. The eggs pass out in animal feces and can persist in soil, sandboxes, or anywhere a pet has defecated. If contaminated dirt gets on your hands and then into your mouth, the larvae can migrate through your body. Young children who play in or eat dirt are at the highest risk for severe infections.
The parasites from pets don’t always behave the same way in humans as they do in animals. Rather than settling into the intestines and growing into adult worms, the larvae sometimes wander through other tissues, causing inflammation in organs or even the eyes. This is why picking up after your pets and deworming them regularly matters for your own health, not just theirs.
How Parasites Survive Your Stomach Acid
Your stomach produces acid strong enough to break down food, so it might seem like parasites shouldn’t be able to survive the trip. Many of them can. Protozoan parasites like Giardia and Entamoeba form cysts with thick, hard walls specifically adapted to withstand harsh environments. These cysts can survive for weeks outside the body and pass through stomach acid essentially unharmed. Once they reach the less acidic environment of the small or large intestine, they “hatch” and begin feeding and reproducing.
Parasite eggs from worms use a similar strategy. The eggshell protects the developing larva until it reaches the intestines, where conditions are right for it to emerge. Larvae consumed in undercooked meat are already past the egg stage but are embedded in muscle tissue, which provides some buffering as it’s digested.
Person-to-Person Spread
Some parasitic infections spread directly between people, particularly in settings with close contact like households, daycare centers, and institutional living. The mechanism is still fecal-oral: microscopic amounts of infected stool transfer from one person to another, often through shared surfaces, inadequate handwashing after using the bathroom, or during diaper changes. Giardia and Cryptosporidium are particularly efficient at spreading this way because their cysts are so small and so resilient.
Reducing Your Risk
Handwashing with soap and water is the single most effective prevention measure. This is one area where hand sanitizer falls short. Alcohol-based sanitizers do not reliably kill parasite cysts like Cryptosporidium. The CDC specifically recommends soap and water over sanitizer for removing these organisms.
Beyond handwashing, the practical steps are straightforward: cook meat to the recommended internal temperatures using a food thermometer, wash fruits and vegetables thoroughly, avoid swallowing water while swimming in natural bodies of water, and treat or filter water from streams or wells before drinking it. If you’re hiking or traveling in areas with uncertain water quality, boiling water for at least one minute is reliable. When gardening or working in soil, wear gloves and wash your hands before eating.
For pet owners, regular veterinary deworming and prompt cleanup of animal waste reduce the parasite load in your yard and home. Keep children’s sandboxes covered when not in use, since outdoor cats commonly use them as litter boxes.