Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic diarrhea, abdominal pain, weight loss, and sometimes vomiting due to chronic inflammation of the gastrointestinal tract.

Irritable Bowel Syndrome (IBS): Although IBS can cause chronic diarrhea and abdominal pain, significant weight loss and vomiting are less common, making it less likely but still a consideration.
Gastrointestinal Infections: Chronic infections such as giardiasis or other parasitic infections can lead to prolonged gastrointestinal symptoms.
Celiac Disease: An autoimmune reaction to gluten can cause chronic diarrhea, abdominal pain, and significant weight loss if left untreated.
Lactose Intolerance or Other Food Intolerances: While less likely to cause significant weight loss, these conditions can lead to chronic gastrointestinal symptoms.

Gastrointestinal Malignancy: Although less common, cancers of the stomach, small intestine, or colon can present with chronic diarrhea, abdominal pain, vomiting, and significant weight loss. Early diagnosis is crucial for treatment.
Neuroendocrine Tumors: Certain tumors like carcinoid syndrome can cause chronic diarrhea and weight loss.
HIV/AIDS: Can present with chronic diarrhea and significant weight loss among other symptoms.
Thyroid Disorders: Hyperthyroidism can cause chronic diarrhea, weight loss, and abdominal symptoms.

Whipple's Disease: A rare bacterial infection that can cause chronic diarrhea, weight loss, and abdominal pain.
Short Bowel Syndrome: A condition where a significant portion of the small intestine is missing or not functioning, leading to malabsorption and chronic diarrhea.
Eosinophilic Gastroenteritis: A rare condition characterized by eosinophilic infiltration of the gastrointestinal tract, leading to chronic gastrointestinal symptoms.
Tropical Sprue: A rare condition seen in certain tropical regions, causing chronic diarrhea and malabsorption.