Several parasites cause diarrhea in humans, but three are responsible for the vast majority of cases worldwide: Giardia, Cryptosporidium, and Entamoeba histolytica. All three are single-celled organisms that infect the gut after being swallowed, typically through contaminated water or food. A smaller number of cases come from worm infections and a lesser-known parasite called Cyclospora. The WHO lists these parasitic infections alongside bacteria and viruses as drivers of nearly 1.7 billion cases of childhood diarrheal disease every year.
Giardia: The Most Common Waterborne Parasite
Giardia is one of the most common causes of waterborne illness in the United States. It infects the small intestine, where heavy infestations damage the lining of the gut wall and interfere with your body’s ability to absorb nutrients. That malabsorption is what drives the diarrhea, along with cramping, bloating, nausea, and foul-smelling loose stools that can look greasy or float.
The incubation period is one to four weeks, which means you may not connect your symptoms to the water you swallowed on a camping trip or the salad you ate while traveling. Before Giardia parasites leave the body in stool, they form hard shells called cysts that let them survive outside the intestines for months. That durability is what makes them so easy to spread through untreated water, contaminated food, and person-to-person contact, particularly in daycare settings and among household members.
Cryptosporidium: Tough to Kill, Tough to Treat
Cryptosporidium (often shortened to “Crypto”) enters the cells lining your intestines by fusing directly with the cell membrane. Once inside, it disrupts normal fluid balance in the gut, causing watery diarrhea that can last one to three weeks. Symptoms typically show up about seven days after exposure, though the window ranges from two to 28 days.
What makes Crypto particularly problematic is its resistance to standard water disinfection. Chlorine, the backbone of most municipal water treatment, does not reliably kill Crypto cysts. That’s why Crypto is the parasite most commonly linked to recreational water outbreaks in pools, water parks, and splash pads. For people with weakened immune systems, the infection can become severe and prolonged because effective treatment options are limited.
Entamoeba Histolytica: When Diarrhea Turns Bloody
Entamoeba histolytica causes amebic dysentery, a form of diarrhea that can include blood and mucus in the stool. Unlike Giardia’s watery, crampy presentation, amebic dysentery tends to come on more gradually with abdominal pain and frequent loose stools that progressively worsen. In severe cases, the parasite can invade the intestinal wall and spread to the liver, forming abscesses.
This parasite is most common in tropical and subtropical regions with poor sanitation. It spreads through fecal contamination of water and food. One important distinction: amebic dysentery can look a lot like bacterial dysentery caused by Shigella, but the causes and treatments are completely different. A stool test is essential to tell them apart, since giving the wrong treatment can delay recovery or cause complications.
Cyclospora: A Foodborne Parasite on the Rise
Cyclospora cayetanensis has gained attention in the U.S. through recurring outbreaks linked to fresh produce, particularly imported berries, herbs, and salad greens. Symptoms start one to 11 days after eating contaminated food and include watery diarrhea, loss of appetite, weight loss, and fatigue. Without treatment, the diarrhea can cycle on and off for weeks or even months.
Risk is highest for people living in or traveling to tropical and subtropical regions where the parasite is endemic, but domestic outbreaks tied to the food supply mean anyone can be exposed.
Worms That Cause Diarrhea
Most people associate parasitic worms with other symptoms, but several helminths do cause diarrhea, particularly with heavier infections. Whipworm (Trichuris trichiura) is the most notable. It burrows into the lining of the large intestine and can cause diarrhea, dysentery, abdominal pain, and in children, impaired growth and chronic malnutrition.
Strongyloides is another worm worth knowing about. It causes gastrointestinal symptoms including diarrhea, and in people with suppressed immune systems it can multiply unchecked inside the body, leading to a dangerous condition called hyperinfection. Both of these worms are soil-transmitted, meaning you pick them up through contact with contaminated soil, usually by walking barefoot or eating unwashed produce grown in affected areas.
How Parasitic Diarrhea Is Diagnosed
Diagnosing a parasitic infection traditionally requires a microscopic examination of stool samples, a process called ova and parasite (O&P) testing. A technician looks for parasite eggs, cysts, or the organisms themselves under a microscope. Because parasites shed intermittently, your doctor may request multiple stool samples collected on different days to improve the chances of detection.
For Giardia and Cryptosporidium specifically, antigen detection tests have largely replaced microscopy as the go-to method. These tests use antibodies to detect parasite proteins in stool and are faster, more sensitive, and don’t require a highly trained microscopist. The CDC considers a direct fluorescent antibody test the current best option for diagnosing Cryptosporidium. Rapid dipstick-style tests also exist for point-of-care settings.
What Treatment Looks Like
Treatment depends entirely on which parasite is causing the problem. Giardia clears with a short course of antiparasitic medication, sometimes as a single dose. Entamoeba histolytica requires a longer treatment course, typically a week to 10 days, followed by a second medication to eliminate any cysts lingering in the intestine. Cyclospora responds well to a specific antibiotic.
Cryptosporidium is the outlier. In healthy adults, the infection is usually self-limiting, meaning your immune system clears it on its own within a few weeks. Medication can shorten the illness but is not always necessary. For young children or immunocompromised individuals, however, Crypto can be much harder to manage, and staying hydrated becomes the most critical concern.
For all parasitic diarrheal infections, dehydration is the immediate risk. Replacing lost fluids and electrolytes matters more in the short term than any medication.
How to Protect Yourself
Since most of these parasites spread through contaminated water or food, prevention centers on what you drink, what you eat, and how you handle hygiene.
- Boiling water is the single most reliable way to kill parasitic cysts, including Crypto and Giardia. A rolling boil for one minute (three minutes at elevations above 6,500 feet) eliminates all parasitic threats.
- Water filters work if you choose carefully. Look for an absolute 1-micron filter or one certified to NSF/ANSI Standard 53 or 58 with “cyst removal” or “cyst reduction” on the label. Nominal 1-micron filters can let 20 to 30 percent of Crypto-sized particles pass through. Ultraviolet light filters and basic carbon filters are not designed to remove these parasites.
- Produce safety matters at home and especially while traveling. Wash fruits and vegetables thoroughly, and in high-risk areas, stick to cooked foods and peelable fruits.
- Hand hygiene is critical, particularly after using the bathroom, changing diapers, and before preparing food. Parasitic cysts are not killed by alcohol-based hand sanitizers, so soap and water is the only effective option.
If you’re traveling to areas with unreliable water treatment, the same rules apply with extra vigilance: avoid ice cubes, brush teeth with bottled or treated water, and skip raw salads and unpeeled fruits at restaurants. These precautions protect against not just parasites but bacterial and viral causes of traveler’s diarrhea as well.