Gastroenteritis: This is a common cause of diarrhea in the elderly, often due to viral or bacterial infections. The elderly are more susceptible due to decreased immune function and potential comorbidities.

Medication-induced diarrhea: Many medications can cause diarrhea as a side effect, and elderly patients often take multiple medications, increasing the risk.
Irritable Bowel Syndrome (IBS): Although IBS can occur at any age, its prevalence and diagnosis might increase with age due to increased awareness and medical evaluation.
Inflammatory Bowel Disease (IBD): While less common in the elderly, Crohn's disease and ulcerative colitis can present at any age and should be considered, especially with chronic diarrhea.
Diverticulitis: Common in the elderly, diverticulitis can cause changes in bowel habits, including diarrhea.

Clostridioides difficile (C. diff) infection: This bacterial infection is a significant concern in the elderly, especially those with recent antibiotic use or hospitalization, due to its high morbidity and mortality.
Ischemic bowel disease: Reduced blood flow to the intestines can cause diarrhea among other symptoms and is critical to diagnose early to prevent severe complications.
Colorectal cancer: Although less likely to present solely with diarrhea, it's a critical diagnosis not to miss, especially in the elderly, due to its potential for significant morbidity and mortality.

Microscopic colitis: A condition characterized by inflammation of the colon that can cause chronic diarrhea, more common in older adults but less common than other causes.
Lactose intolerance or other malabsorption syndromes: These can cause chronic diarrhea but are less common in the elderly as a new onset condition.
Thyroid disorders: Hyperthyroidism can cause diarrhea, although it's a less common presentation, especially in the elderly.
Neuroendocrine tumors: Rare tumors that can produce substances leading to diarrhea, such as VIPomas.