Parasites
Giardiasis is caused by microscopic parasites known as giardia intensinalis. The parasites live in the intestines of humans and animals.
In most cases, the parasites cause no symptoms, and many people have no idea that they are infected.
It is estimated that in parts of the world where giardiasis is widespread (see Prevention for a list), 20-30% of the entire population could be infected.
How is giardiasis spread?
When an infected person passes a stool (goes to the toilet), some of the parasites inside the intestines can be passed out of the body inside the stool.
After the parasites have left the body, they form a hard, protective shell that helps to protect them from the environment. In this form, the parasite is known as a giardia cyst. Giardia cysts can survive outside the body for several months or sometimes several years.
Once outside the body, giardiasis is usually spread by drinking water that has been contaminated with infected stools (faeces). This most commonly occurs in countries that have poor sanitation and limited access to clean water.
Less commonly, giardiasis is spread when an infected person does not wash their hands properly after going to the toilet, and then transfers the parasites onto surfaces, utensils or food.
Anyone who touches infected surfaces, uses infected utensils, or eats contaminated food can transfer the parasites into their mouth and cause an infection.
Other risk factors
Parents or childcare workers who are changing the nappy of a baby with giardiasis also have an increased risk of developing the condition by accidentally transferring infecting faecal matter into their mouth.
There have been a number of cases where hikers and campers have contracted giardiasis after drinking contaminated water from streams and lakes. You should never drink untreated water (water that has not been boiled or chemically treated) even if it looks clean.
Giardiasis can also be spread during anal sex. Anal sex often involves one person touching the anus (the inside of the bottom) of another person. If you have touched the anus of someone infected with the giardiasis parasite, and you then touch your mouth before washing your hands, you may become infected.
