Viral Gastroenteritis: This is the most common cause of the combination of body aches, fatigue, and diarrhea, often resulting from viral infections such as norovirus or rotavirus. The symptoms typically resolve on their own within a few days.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic diarrhea, fatigue, and body aches due to inflammation in the gastrointestinal tract.
Irritable Bowel Syndrome (IBS): While IBS can cause significant discomfort, it typically does not lead to severe body aches but can cause fatigue and changes in bowel habits, including diarrhea.
Food Poisoning: Bacterial or parasitic infections from contaminated food can lead to acute onset of diarrhea, body aches, and fatigue.

Infectious Colitis: Infections like Clostridioides difficile (C. diff), Salmonella, or Shigella can cause severe diarrhea and potentially life-threatening complications if not treated promptly.
Appendicitis: Although more commonly associated with abdominal pain, appendicitis can sometimes present with diarrhea and body aches, especially if the appendix is located near the rectum.
HIV Infection: Acute HIV infection can present with flu-like symptoms including body aches, fatigue, and gastrointestinal symptoms like diarrhea.

Whipple's Disease: A rare, systemic bacterial infection that can cause diarrhea, weight loss, and body aches, among other symptoms.
Tropical Sprue: A rare condition that affects the small intestine's ability to absorb nutrients, leading to chronic diarrhea, fatigue, and weight loss, typically seen in individuals who have traveled to or live in tropical regions.
Giardiasis: An intestinal infection caused by the parasite Giardia lamblia, which can lead to chronic diarrhea, fatigue, and weight loss if not treated.