What organism is commonly transmitted through contaminated water and causes diarrhea?

Giardia lamblia is a flagellated protozoan that is well-known for causing gastrointestinal illness, particularly diarrhea, due to its transmission through contaminated water sources. The organism can persist in cold water for long periods and is often found in areas where sanitation is inadequate or where water may be contaminated by fecal matter from infected individuals or animals.

After ingestion, Giardia can infect the intestines, leading to symptoms including diarrhea, abdominal cramps, and bloating. The association with water is strong, especially in developing regions or among individuals who consume untreated water from natural sources.

Although other organisms listed can also cause gastrointestinal issues, Giardia lamblia specifically stands out for its clear link to waterborne transmission and consequent diarrhea, making it the most common culprit in this context.