Viral Gastroenteritis: This is often the most common cause of acute diarrhea in adults, including the elderly. Viruses such as norovirus and rotavirus are highly contagious and can spread quickly, especially in closed environments like nursing homes or cruise ships.

Bacterial Gastroenteritis: Bacteria like Salmonella, Shigella, and Campylobacter can cause diarrhea. This could be from contaminated food or water.
Medication-induced Diarrhea: Many medications, including antibiotics, can cause diarrhea as a side effect. Given the patient's age, polypharmacy is a consideration.
Inflammatory Bowel Disease (IBD) Flare: Conditions like Crohn's disease and ulcerative colitis can cause chronic diarrhea, and a flare could present acutely.
Irritable Bowel Syndrome (IBS): Although more chronic, an exacerbation of IBS could present with acute diarrhea.

Clostridioides difficile (C. diff) Infection: Especially in elderly patients who have recently been hospitalized or exposed to antibiotics, C. diff can cause severe, life-threatening diarrhea.
Ischemic Bowel: Reduced blood flow to the intestines can cause acute diarrhea, among other symptoms, and is a medical emergency.
Infectious Colitis: Such as from E. coli O157:H7, which can lead to hemolytic uremic syndrome (HUS), a potentially fatal complication.
Food Poisoning: Certain types, like from Staphylococcus aureus or Bacillus cereus, can cause severe, acute symptoms.

Microscopic Colitis: A condition characterized by chronic, watery diarrhea, more common in older adults but less common than other causes.
Whipple's Disease: A rare, systemic bacterial infection that can cause diarrhea, among many other symptoms.
Lactose Intolerance or Other Carbohydrate Malabsorption: Though not typically acute, could be considered if other causes are ruled out and there's a history suggestive of malabsorption.
Neuroendocrine Tumors: Certain tumors, like VIPomas, can cause severe, chronic diarrhea, but these are exceedingly rare.