Giardia lamblia is a tiny, single-celled parasite that can cause an infection known as giardiasis. It lives in the intestines of humans and animals, often spreading through contaminated water or food. People infected with Giardia lamblia may experience symptoms like diarrhea, stomach cramps, and nausea, which can last for weeks if not treated. This parasite is particularly common in areas with poor sanitation and can be found in lakes, rivers, and even swimming pools. To prevent infection, it's important to drink clean water, wash hands regularly, and avoid consuming food that may be contaminated with Giardia lamblia.