Giardia Infection: Causes and How It Spreads

Key points

  • You can get sick if you swallow Giardia germs.
  • When people or animals are infected, Giardia germs leave the body through poop. Anything that gets this poop on it can spread Giardia to someone else.
  • Giardia spreads easily from one person to another or through water, food, surfaces, or objects.
Illustration of Giardia parasites


Giardia infections are caused by the parasite Giardia duodenalis (Giardia for short). A parasite is a type of germ that gets its food from, or at the expense of, its host. Parasites live on or in a host (for example, a person or animal).

Giardia is protected by an outer shell called a cyst. This outer shell allows it to survive outside the body for weeks to months (for example, in soil or water). It also helps protect Giardia from being killed by chlorine disinfectants like bleach.

Giardia lifecycle

Giardia can be in water or food, or on hands, surfaces, or objects. You can get sick when you swallow Giardia germs.

Once the Giardia germs are in your small intestine (part of your gut), they shed their shell and split in two. These Giardia germs start absorbing nutrients from you.

Giardia germs can continue to multiply in your small intestine.

The germs then move to your colon (lower gut) and regain their shell.

Giardia leaves your body through poop. The Giardia in your poop can spread to other people and make them sick.

Scanning electron microscopic image of a Giardia parasite over an intestinal cell. The parasite has a disc on its underside and moving hair-like structures.
Giardia (shown here under a microscope floating over a cell in the intestine) can attach to your gut to absorb nutrients.

Dr. Stan Erlandsen

Keep Reading: Giardiasis

How it spreads

Giardia spreads easily; swallowing just a few Giardia germs can make you sick.

Giardia germs are in poop of people or animals who are infected, so anything that gets contaminated by human or animal poop can potentially spread the germs. Giardia can spread from one person to another. It can also spread through contaminated water, food, surfaces, or objects.

You can get sick from Giardia by:

  • Having close contact with someone who is sick from Giardia, especially in childcare settings
  • Swallowing water contaminated with Giardia, such as lake, pool, or unsafe drinking water
  • Swallowing food contaminated with Giardia
  • Having contact with poop during sex with someone who is sick, or was recently sick, from Giardia
  • Getting Giardia in your mouth that you picked up from contaminated surfaces, such as bathroom handles or toys
  • Touching animals that have a Giardia infection or animal environments that are contaminated with poop

Anyone can get sick from Giardia, but children get sick more often than adults. Learn if you may be more likely to get sick.

If your pet has a Giardia infection‎

Although animals can spread Giardia to people, you are unlikely to get a Giardia infection from dogs or cats. The type of Giardia that makes people sick is usually not the same type that makes dogs and cats sick.

If you have a private well

If poop gets into the water supply for your private well, your water can get contaminated with Giardia and other germs. This can happen from:

  • Sewage overflows
  • Nearby septic or other sewage systems that are not working correctly
  • Polluted flood or storm water

Your well may be more likely to get contaminated with poop and germs after flooding if it is:

  • Shallow
  • Bored or dug
  • Covered with floodwater for a long time

Learn how to test your well to make sure your water is safe to drink.