Viral Gastroenteritis: This is the most common cause of watery diarrhea, often due to norovirus or rotavirus, and typically presents with sudden onset of diarrhea, vomiting, and abdominal cramps.

Bacterial Gastroenteritis: Caused by bacteria such as E. coli, Salmonella, or Campylobacter, which can lead to watery diarrhea, often accompanied by fever and abdominal pain.
Food Poisoning: Consuming contaminated food can lead to watery diarrhea, usually within hours of ingestion, and may be accompanied by vomiting and stomach cramps.
Lactose Intolerance: Inability to digest lactose can cause watery diarrhea after consuming dairy products, along with bloating and gas.
Irritable Bowel Syndrome (IBS): A chronic condition that can cause watery diarrhea, abdominal pain, and changes in bowel habits, though the exact cause is unknown.

Cholera: A bacterial infection that causes severe, watery diarrhea (often described as "rice-water stool"), which can lead to rapid dehydration and death if not treated promptly.
Clostridioides difficile (C. diff) Infection: A bacterial infection that can cause severe, watery diarrhea, especially after antibiotic use, and can lead to life-threatening complications if not treated.
Giardiasis: An intestinal infection caused by the Giardia parasite, which can lead to chronic watery diarrhea, weight loss, and malabsorption if left untreated.

Whipple's Disease: A rare, systemic bacterial infection that can cause chronic watery diarrhea, weight loss, and malabsorption, among other symptoms.
Microscopic Colitis: A condition characterized by chronic, watery diarrhea, often without visible inflammation on endoscopy, and can be associated with various medications and conditions.
VIPoma: A rare pancreatic tumor that secretes vasoactive intestinal peptide (VIP), leading to severe, watery diarrhea, hypokalemia, and achlorhydria.