Giardia spreads through the fecal-oral route, meaning you swallow microscopic parasites that originated in the poop of an infected person or animal. It takes as few as 10 cysts (the dormant, shell-like form of the parasite) to cause an infection, which is why Giardia passes so easily through contaminated water, food, surfaces, and direct contact with infected individuals.
How the Parasite Works
Giardia exists in two forms. Outside the body, it survives as a cyst, a tough, protective shell that makes it remarkably durable in the environment. Once you swallow cysts and they reach your small intestine, each one opens up and releases two active parasites called trophozoites. These attach to the lining of the intestine, reproduce, and eventually form new cysts that pass out in your stool, ready to infect someone else.
The cyst stage is what makes Giardia so contagious. Cysts survive for two to three months in cold water below 50°F (10°C), and they remain viable for almost a month even in warmer water around 70°F (21°C). Heat is their weakness: water temperatures of about 130°F (54°C) kill them in 10 minutes.
Contaminated Water
Water is the most common vehicle for Giardia transmission. Lakes, rivers, streams, and untreated wells can all harbor cysts from animal or human waste. Swallowing even a small amount of contaminated water while swimming, kayaking, or drinking from a stream is enough to cause infection.
Swimming pools and water parks are not necessarily safe either. Giardia cysts are resistant to the chlorine levels used in standard pool disinfection. Much higher doses of chlorine than what pools typically maintain would be needed to reliably kill the parasite, which is why pool-related outbreaks happen. If someone with an active infection swims, they can release cysts into the water that survive normal treatment.
For drinking water in backcountry or travel settings, a filter with a pore size of 1 micron or smaller will physically remove Giardia cysts. Look for filters certified to NSF/ANSI Standard 53 or 58 with labeling that says “cyst removal” or “cyst reduction.” Boiling water for at least one minute also kills the parasite effectively.
Person-to-Person Contact
Direct contact is the second major route. Because the infective dose is so low, even tiny amounts of fecal contamination on hands, shared objects, or surfaces can transmit the parasite. This is especially common in childcare settings, where diaper changes happen frequently and young children put their hands and objects in their mouths. Children under 5 have one of the highest infection rates of any age group, at about 7 cases per 100,000 people.
Sexual contact that involves exposure to fecal matter is another documented route. Giardia can also spread through contaminated surfaces like bathroom handles, changing tables, and toys. The CDC has confirmed detection of Giardia on stainless steel and laminate surfaces in daycare centers.
Food Contamination
Food can carry Giardia when it’s been washed in contaminated water, handled by someone with an active infection who didn’t wash their hands, or grown in soil contaminated with infected feces. Raw produce is the most likely culprit, since cooking at high temperatures destroys the cysts. This route is less common than waterborne transmission in the United States but plays a larger role in regions with less reliable water treatment and sanitation infrastructure.
Animals and Giardia
Despite what many pet owners worry about, you’re unlikely to catch Giardia from your dog or cat. The strains of Giardia that infect dogs and cats are generally not the same strains that infect people. However, some types of Giardia do cross between species. Beavers, chinchillas, birds, opossums, and monkeys can carry strains that are transmissible to humans. Beavers in particular have long been associated with contaminating mountain streams, which is why Giardia infection is sometimes called “beaver fever.”
Contact with animal feces or environments contaminated with animal waste (farms, petting zoos, wildlife areas) still carries some risk, even if household pets are low on the list.
Who Gets Infected Most Often
In the U.S., about 13,800 cases of giardiasis were reported in 2022, an 18% increase over the previous year. The actual number of infections is almost certainly higher, since many cases go undiagnosed or unreported. Infection rates are highest in the Northeast (6.8 per 100,000 people) and lowest in the South (4.2 per 100,000). Men account for about 62% of reported cases. The two age groups with the highest rates are children under 5 and adults aged 30 to 34.
The pattern makes sense given the transmission routes. Young children in group care settings pass the parasite easily through close contact. Adults in their early 30s may face higher exposure through outdoor recreation, international travel, or sexual contact.
Practical Steps to Avoid Infection
Handwashing with soap and water is the single most effective prevention measure, particularly after using the bathroom, changing diapers, and before preparing food. Alcohol-based hand sanitizers do not reliably kill Giardia cysts, so soap and water is the better choice when dealing with potential fecal contamination.
When swimming, avoid swallowing pool, lake, or river water. If your child has diarrhea, keep them out of swimming areas and away from daycare until the diarrhea has fully stopped. In childcare settings, regularly cleaning and disinfecting toys, changing areas, and shared surfaces reduces transmission.
For hikers, campers, and travelers in areas with uncertain water quality, filtering or boiling water before drinking is essential. Simply letting water sit or adding standard purification tablets may not be enough, given how resistant Giardia cysts are to chemical treatment at typical concentrations.