Giardia passes through several stages and forms, and exists primarily in a cyst form when ingested. In contrast to bacterial infections where it may take exposure to thousands of bacteria to cause disease, it only takes ingestion of one giardia cyst to become infected, so even minimal exposure to contaminated water can lead to infection. Particularly foul smelling stools, and much bloating, are common. There is usually no fever, and the diarrhea is often episodic rather than acutely severe. The symptoms may last for weeks, and weight loss can occur in many people infected with the parasite.
Loss of appetite is common.
Anyone can get giardiasis, although there is an increased risk of infection in children and those who may have a compromised immune system. Even though children may be more likely to acquire the acute infection, chronic infection is more common in adults. Definite diagnosis can be made by a laboratory test in which the microscopic forms of the parasite are seen in stool samples. More exotic tests are available but are expensive. The usual adult dose is 250 mg tablets three times daily for 7-10 days.