Viral Gastroenteritis: This is often the most common cause of diarrhea lasting more than a week, especially if the onset was sudden. Viruses such as norovirus, rotavirus, and adenovirus are common culprits.

Bacterial Gastroenteritis: Bacteria like Salmonella, Shigella, and Campylobacter can cause prolonged diarrhea. The history of exposure to undercooked food, travel, or contact with someone who has similar symptoms can support this diagnosis.
Irritable Bowel Syndrome (IBS): Although IBS is a chronic condition, its onset can sometimes be triggered by an episode of gastroenteritis. It's characterized by abdominal pain, changes in bowel habits, and the absence of other alarming symptoms.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can present with chronic diarrhea, abdominal pain, weight loss, and sometimes blood in the stool.

Clostridioides difficile (C. diff) Infection: This bacterial infection can cause severe, prolonged diarrhea, especially in individuals who have recently used antibiotics. It's crucial to consider this diagnosis due to its potential severity and the need for specific treatment.
Giardiasis: An infection caused by the Giardia parasite, often acquired through contaminated water or food. It can lead to chronic diarrhea, weight loss, and malabsorption if not treated.
HIV Infection: Diarrhea can be a symptom of acute HIV infection or a sign of the progression to AIDS, where the immune system is severely compromised, leading to opportunistic infections.

Whipple's Disease: A rare, systemic bacterial infection (Tropheryma whipplei) that can cause chronic diarrhea, weight loss, arthralgias, and neurological symptoms.
Lactose Intolerance or Other Carbohydrate Malabsorption: Conditions where the body cannot digest certain sugars, leading to chronic diarrhea, bloating, and gas.
Microscopic Colitis: A condition characterized by chronic, watery diarrhea, often without blood or abdominal pain, diagnosed by microscopic examination of colon biopsy specimens.
Thyroid Disorders: Both hyperthyroidism and hypothyroidism can cause changes in bowel habits, including diarrhea, though this is less common and usually accompanied by other systemic symptoms.