Viral Gastroenteritis: This is the most common cause of fever and diarrhea in infants, especially during the late summer months when viruses such as norovirus and rotavirus are prevalent. The symptoms typically include watery diarrhea, vomiting, fever, and abdominal cramps.

Bacterial Gastroenteritis: Caused by bacteria like Salmonella, Shigella, or Campylobacter, which can be contracted through contaminated food or water. Symptoms are similar to viral gastroenteritis but may be more severe.
Food Poisoning: Can occur from consuming contaminated or spoiled food, leading to symptoms like diarrhea, fever, and vomiting.
Intestinal Parasites: Though less common, parasites like Giardia can cause gastrointestinal symptoms, especially in children who drink untreated water or engage in activities that increase exposure risk.

Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially cutting off blood flow. It's a medical emergency that can present with intermittent abdominal pain, vomiting, and sometimes diarrhea, especially if there's intestinal obstruction.
Appendicitis: Although less common in infants, appendicitis can present with fever, vomiting, and abdominal pain that may be accompanied by diarrhea.
Urinary Tract Infection (UTI): Can cause fever and abdominal discomfort, and if the infection involves the kidneys (pyelonephritis), it may lead to more severe symptoms.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis are rare in infants but can cause chronic diarrhea, fever, and abdominal pain.

Hemolytic Uremic Syndrome (HUS): A rare condition that can occur after certain bacterial infections (like E. coli O157:H7), leading to kidney failure and other severe complications.
Congenital or Acquired Immunodeficiency: Infants with immunodeficiency may be more susceptible to opportunistic infections that can cause prolonged or severe gastrointestinal symptoms.
Toddlers' Diarrhea (Chronic Non-Specific Diarrhea): A condition of unknown cause characterized by chronic diarrhea in otherwise healthy toddlers, which might be considered if other diagnoses are ruled out.