Some people with giardia infection never develop signs or symptoms but still carry the parasite and can spread it to others through their stool. For those who do get sick, signs and symptoms usually appear one to three weeks after exposure and may include:

• Watery, sometimes foul-smelling diarrhea that may alternate with soft, greasy stools
• Fatigue or malaise
• Abdominal cramps and bloating
• Gas or flatulence
• Nausea
• Weight loss

Signs and symptoms of giardia infection may last two to six weeks, but in some people they last longer or recur.

Giardia parasites live in the intestines of people and animals. Before the microscopic parasites are passed in stool, they become encased within hard shells called cysts, which allows them to survive outside the intestines for months. Once inside a host, the cysts dissolve and the parasites are released.

Infection occurs when you accidentally ingest the parasite cysts. This can occur by swallowing contaminated water, by eating contaminated food or through person-to-person contact.

Swallowing contaminated water

The most common way to become infected with giardia is after swallowing contaminated water. Giardia parasites are found in lakes, ponds, rivers and streams worldwide, as well as in municipal water supplies, wells, cisterns, swimming pools, water parks and spas. Ground and surface water can become contaminated from agricultural runoff, wastewater discharge or animal feces. Children in diapers and people with diarrhea may accidentally contaminate pools and spas.

Eating contaminated food

Giardia parasites can be transmitted through food — either because food handlers with giardiasis don't wash their hands thoroughly or because raw produce is irrigated or washed with contaminated water. Because cooking food kills giardia, food is a less common source of infection than water is, especially in industrialized countries.

Person-to-person contact

You can contract giardiasis if your hands become contaminated with fecal matter — parents changing a child's diapers are especially at risk. So are child care workers and children in child care centers, where outbreaks are increasingly common. The giardia parasite can also spread through anal sex.

The giardia parasite is a very common intestinal parasite. Although anyone can pick up giardia parasites, some people are especially at risk:

• Children. Giardia infection is far more common in children than it is in adults. Children are more likely to come in contact with feces, especially if they wear diapers, are toilet training or spend time in a child care center. People who live or work with small children also are at higher risk of developing giardia infection.
• People without access to safe drinking water. Giardiasis is rampant wherever sanitation is inadequate or water isn't safe to drink. You're at risk if you travel to places where giardiasis is common, especially if you aren't careful about what you eat and drink. The risk is greatest in rural or wilderness areas.
• People who have anal sex. Having anal sex without using a condom puts you at increased risk of giardia infection, as well as sexually transmitted infections.

Giardia infection is almost never fatal in industrialized countries, but it can cause lingering symptoms and serious complications, especially in infants and children. The most common complications include:

• Dehydration. Often a result of severe diarrhea, dehydration occurs when the body doesn't have enough water to carry out its normal functions.
• Failure to thrive. Chronic diarrhea from giardia infection can lead to malnutrition and harm children's physical and mental development.
• Lactose intolerance. Many people with giardia infection develop lactose intolerance — the inability to properly digest milk sugar. The problem may persist long after the infection has cleared.