Giardiasis is caused by microscopic parasites known as Giardia intestinalis. The parasites live in the intestines of humans and animals. In most cases, the infection is caught from other humans.
The parasites don't usually cause any symptoms, and people have no idea they're infected. In parts of the world where giardiasis is widespread, an estimated one in five people could be infected.
How giardiasis is spread
While inside the intestines, the parasites form a hard protective shell known as a giardia cyst.
When someone with the giardiasis infection goes to the toilet, some of the cysts inside the intestines can be passed out of the body inside faeces. Giardia cysts can survive outside the body for several weeks or months.
Once outside the body, giardiasis is usually spread by drinking water that's been contaminated with infected faeces. This most commonly occurs in countries with poor sanitation and limited access to clean water.
Giardiasis can also be spread through direct contact between people. Less commonly, giardiasis is spread when an infected person doesn't wash their hands properly after going to the toilet and transfers the parasites on to surfaces, utensils or food.
Who's at risk?
Parents or childcare workers who change the nappy of a baby with giardiasis have an increased risk of developing the condition by accidentally transferring faeces into their mouth. The risk is higher in places where there are many babies and frequent nappy changing, such as day care centres and nurseries.
There have been cases of hikers and campers developing giardiasis after drinking contaminated water from streams and lakes. You should always avoid drinking untreated water (water that hasn't been boiled or chemically treated), even if it looks clean.
A small number of outbreaks of giardiasis have been linked to recreational water areas, such as water parks and swimming pools, that have become contaminated with giardiasis parasites.
People travelling to parts of the world where standards of water hygiene are poor have an increased risk of developing giardiasis. However, because of the time it takes for symptoms to appear after becoming infected, most people won't have any symptoms until they return home.
People who have regular anal sex are also at an increased risk of contracting giardiasis, as the giardia parasite can be passed from the anus (back passage) to the mouth during sexual intercourse.
Read about preventing giardiasis for advice about reducing your risk of developing the condition.