Viral Gastroenteritis: This is the most common cause of diarrhea, often resulting from viral infections such as norovirus or rotavirus, which are highly contagious and can spread through contaminated food, water, or close contact with an infected person.

Bacterial Gastroenteritis: Caused by bacteria like Salmonella, E. coli, or Campylobacter, often from consuming contaminated food or water.
Food Poisoning: Similar to bacterial gastroenteritis but typically implies a more acute onset after consuming specific contaminated foods.
Irritable Bowel Syndrome (IBS): A chronic condition characterized by recurring abdominal pain associated with a change in bowel habits, which can include diarrhea.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic diarrhea, among other symptoms.

Clostridioides difficile (C. diff) Infection: A potentially life-threatening condition that can cause severe diarrhea, especially in individuals who have recently used antibiotics.
Cholera: Although rare in many parts of the world, cholera is a highly infectious disease that causes severe, watery diarrhea and can lead to rapid dehydration and death if not treated promptly.
Giardiasis: An intestinal infection caused by the parasite Giardia lamblia, which can lead to severe diarrhea and malabsorption if left untreated.

Whipple's Disease: A rare, systemic bacterial infection that can cause diarrhea, among other symptoms like weight loss and joint pain.
Lactose Intolerance: The inability to digest lactose, a sugar found in milk, leading to diarrhea after consuming lactose-containing products.
Short Bowel Syndrome: A condition that can occur after a significant portion of the small intestine is removed or is not functioning, leading to malabsorption and chronic diarrhea.
Microscopic Colitis: A type of inflammation of the colon that can cause chronic, watery diarrhea, often in older adults.