Irritable Bowel Syndrome (IBS): This is a common condition characterized by chronic abdominal pain, bloating, and alteration of bowel habits in the absence of any mechanical, inflammatory, or biochemical abnormalities. The age and symptom of frequent diarrhea without other alarming signs make IBS a plausible first consideration.

Infectious Gastroenteritis: This could be due to viral, bacterial, or parasitic infections. The diagnosis is likely if the patient has recently traveled, has been exposed to someone with similar symptoms, or has consumed undercooked or raw foods.
Lactose Intolerance: A condition where the body is unable to digest lactose, a sugar found in milk, leading to diarrhea, gas, and bloating after consuming lactose-containing products.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic diarrhea, abdominal pain, and weight loss. Although less common than IBS, they are important to consider due to their significant impact on quality of life and potential for complications.
Food Intolerances or Sensitivities: Such as gluten sensitivity or celiac disease, which can cause diarrhea among other symptoms after consuming specific foods.

Celiac Disease: An autoimmune disorder where the ingestion of gluten leads to damage in the small intestine. It's crucial to diagnose due to the risk of malnutrition and increased risk of other autoimmune diseases if left untreated.
Giardiasis: A parasitic infection that can cause chronic diarrhea, especially in individuals who have traveled to areas with poor sanitation or have been exposed to contaminated water.
HIV Infection: Diarrhea can be a symptom of acute HIV infection or a sign of immunocompromised status in later stages, making it critical to consider in the differential diagnosis, especially if other risk factors are present.
Thyroid Disorders: Hyperthyroidism can cause increased bowel movements and diarrhea, among other symptoms.

Whipple's Disease: A rare, systemic bacterial infection that can cause diarrhea, weight loss, and arthralgias.
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.
VIPoma: A rare endocrine tumor that secretes vasoactive intestinal peptide (VIP), causing a syndrome of watery diarrhea, hypokalemia, and achlorhydria.
Microscopic Colitis: A condition characterized by chronic, watery diarrhea, often without visible abnormalities on colonoscopy but with distinct microscopic changes.