Viral Gastroenteritis: This is the most common cause of the symptoms described, often due to norovirus or rotavirus, leading to inflammation of the stomach and intestines, resulting in nausea, vomiting, diarrhea, and fever.

Food Poisoning: Bacterial infections from contaminated food can cause similar symptoms, with common culprits including Salmonella, E. coli, and Staphylococcus aureus.
Inflammatory Bowel Disease (IBD) Flare: Conditions like Crohn's disease and ulcerative colitis can present with these symptoms during a flare-up.
Gastroesophageal Reflux Disease (GERD): Although less likely to cause diarrhea, severe GERD can lead to nausea, vomiting, and fever if there is an associated infection or esophagitis.

Appendicitis: While the classic presentation includes abdominal pain, nausea, vomiting, and fever, diarrhea can occasionally be present, especially if the appendix is near the intestines.
Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause bowel obstruction and ischemia, presenting with vomiting, diarrhea, and fever.
Mesenteric Ischemia: Reduced blood flow to the intestines can cause severe abdominal pain, nausea, vomiting, diarrhea, and fever, requiring immediate medical attention.

Whipple's Disease: A rare, systemic bacterial infection that can cause diarrhea, weight loss, and fever, among other symptoms.
Tropical Sprue: A rare condition affecting the small intestine's ability to absorb nutrients, often presenting with chronic diarrhea, weight loss, and malabsorption.
Giardiasis: An intestinal infection caused by the Giardia parasite, which can lead to diarrhea, abdominal cramps, and dehydration, especially in travelers or those exposed to contaminated water.