You get giardia by swallowing the parasite’s microscopic cysts, almost always through contaminated water, contaminated food, or direct contact with infected feces. The infection is remarkably easy to catch. Swallowing just a few cysts is enough to make you sick, and more than 280 million cases occur worldwide each year.
The Fecal-Oral Route
Every giardia infection starts the same way: cysts from infected feces end up in your mouth. This is called fecal-oral transmission, and it happens more easily than most people expect. The cysts are shed in the stool of infected humans and animals, and they’re tough enough to survive outside the body for weeks or months. Anything that comes into contact with contaminated feces, whether water, food, surfaces, or hands, can carry the parasite to its next host.
Once you swallow the cysts, they travel to your small intestine, where stomach acid triggers them to open and release the active form of the parasite. Symptoms typically appear after an incubation period of 1 to 14 days, with 7 days being the average. That lag means you can be exposed on a camping trip and not feel anything until well after you’re home.
Contaminated Water
Water is the single most common source of giardia infections. Lakes, rivers, streams, and ponds can all harbor cysts from the feces of infected wildlife or humans. Swallowing even a small amount of untreated water while swimming, kayaking, or wading is enough. Swimming pools, splash pads, and water parks are also sources, because giardia cysts are resistant to the chlorine levels used in most recreational water settings.
Giardia cysts are remarkably durable in the environment. In water colder than about 50°F (10°C), cysts can survive for two to three months. Even at room temperature, they remain viable for nearly a month. This means a stream that looks perfectly clean can carry infectious cysts long after the source of contamination has moved on.
Unsafe drinking water is a major route of infection in parts of the world where water treatment infrastructure is limited. But even in countries with modern water systems, untreated well water or disruptions to municipal treatment can lead to outbreaks.
Food and Infected Handlers
Food becomes contaminated with giardia in two main ways. The first is at the farm level: produce irrigated with contaminated water, or washed in contaminated water during packing, can carry cysts on its surface. Raw fruits and vegetables are the biggest concern because cooking kills the parasite. The second route is through food handlers. A person infected with giardia who doesn’t wash their hands thoroughly after using the bathroom can transfer cysts to anything they prepare or serve.
Person-to-Person Spread
Close contact with an infected person is another common route, particularly in settings where hygiene is difficult to maintain perfectly. Childcare centers are a well-known hotspot. Young children in diapers can shed large numbers of cysts, and the parasite spreads during diaper changes, through shared toys, and on contaminated surfaces like changing tables and bathroom handles. From there, it easily moves to caregivers and family members.
Sexual contact that involves exposure to feces is another recognized route. This includes oral-anal contact or any sexual activity where trace amounts of stool could be transferred. The CDC specifically lists contact with feces during sex with someone who is currently or recently infected as a way giardia spreads.
Animals and Pets
Giardia infects a wide range of animals, including dogs, cats, beavers, and livestock. You can pick up the parasite by touching infected animals or by contact with environments contaminated with their feces, such as farm areas or yards. That said, the risk from household pets is lower than many people assume. The strains of giardia that typically infect dogs and cats are usually not the same strains that cause illness in humans. Transmission from pets to owners is possible but unlikely.
Wildlife is a different story. Beavers, muskrats, and other animals that live near freshwater sources regularly shed giardia cysts into streams and lakes, which is why giardiasis has sometimes been called “beaver fever.” If you’re hiking or camping near waterways frequented by wildlife, the water should be treated before drinking.
Surfaces and Objects
Because giardia cysts are shed in stool and can persist in the environment, contaminated surfaces play a real role in transmission. Bathroom door handles, diaper-changing areas, toys in childcare settings, and shared objects in institutional environments can all harbor cysts. You pick them up on your hands and inadvertently transfer them to your mouth. This is why handwashing with soap and water at key moments, especially after using the bathroom and after changing diapers, is the most effective way to break the chain of transmission.
How to Protect Yourself
If you’re hiking, camping, or traveling somewhere with uncertain water quality, you have a few reliable options. Bringing water to a rolling boil for one minute kills giardia cysts (extend that to three minutes above 6,500 feet elevation). Portable water filters also work, but the filter needs an absolute pore size of 1 micron or smaller, or certification under NSF standards 53 or 58 for cyst removal. Chemical disinfectants like iodine tablets are less reliable against giardia than boiling or filtration.
In everyday life, the basics matter most. Wash your hands with soap and water after using the bathroom, before preparing food, and after changing diapers. Avoid swallowing water when swimming in lakes, rivers, or pools. If you’re caring for children in group settings, clean and sanitize toys and surfaces regularly, and change diapers away from food preparation areas and water sources. When traveling internationally, stick to bottled or treated water and avoid raw produce that may have been washed in local water.