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 Intolerance: Conditions like lactose intolerance, where the body cannot digest certain nutrients, leading to diarrhea.
Irritable Bowel Syndrome (IBS): A chronic condition characterized by recurring abdominal pain and changes in bowel movements, including diarrhea.
Medication Side Effects: Certain medications, such as antibiotics, can cause diarrhea as a side effect.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis, which are chronic and can lead to severe complications if not treated.
Celiac Disease: An autoimmune disorder where the ingestion of gluten leads to damage in the small intestine, causing diarrhea among other symptoms.
Giardiasis: An infection caused by the Giardia parasite, which can lead to severe diarrhea and dehydration if not properly treated.
Clostridioides difficile (C. diff) Infection: A bacterial infection that can cause severe diarrhea and is often associated with antibiotic use.

Whipple's Disease: A rare, systemic bacterial infection that can cause diarrhea, among other symptoms like weight loss and joint pain.
Short Bowel Syndrome: A condition where a significant portion of the small intestine is missing or not functioning, leading to malabsorption and diarrhea.
Microscopic Colitis: A condition characterized by inflammation of the colon, leading to chronic diarrhea.
Tropical Sprue: A rare condition causing malabsorption and diarrhea, typically acquired in tropical regions.