Giardia lamblia is a microscopic parasite that causes a gastrointestinal infection known as giardiasis. It is commonly found in contaminated water sources, such as lakes and streams, and can also be transmitted through food or person-to-person contact. Symptoms of giardiasis include diarrhea, abdominal cramps, and nausea. The parasite attaches to the intestinal lining, disrupting nutrient absorption and leading to digestive issues. Diagnosis is typically made through stool tests, and treatment often involves medications like metronidazole or tinidazole. Preventive measures include practicing good hygiene and ensuring safe drinking water.