Viral Gastroenteritis: This is the most common cause of vomiting and diarrhea in infants, often due to rotavirus or norovirus, and typically presents without fever in the early stages.

Overfeeding or Underfeeding: Incorrect feeding techniques or volumes can lead to gastrointestinal upset in infants.
Lactose Intolerance: Though less common in infants, especially at this age, it can cause similar symptoms.
Gastroesophageal Reflux Disease (GERD): Can cause vomiting, especially after feeding, and is common in infants.
Infectious Causes Other Than Viral Gastroenteritis: Bacterial infections like Salmonella or E. coli, though less common in this age group without exposure history.

Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause intermittent vomiting and diarrhea, and is a surgical emergency.
Volvolus or Intestinal Obstruction: Conditions that can cause bowel obstruction, leading to severe vomiting and potentially life-threatening if not promptly treated.
Congenital Anomalies: Such as pyloric stenosis, which can cause projectile vomiting and is a surgical emergency.
Metabolic Disorders: Certain metabolic disorders can present with vomiting and diarrhea, and early diagnosis is crucial for management.

Food Protein-Induced Enterocolitis Syndrome (FPIES): A rare condition causing vomiting and diarrhea after ingestion of specific food proteins.
Congenital Infections: Such as congenital CMV infection, which can cause gastrointestinal symptoms among other systemic findings.
Immunodeficiency: Certain immunodeficiencies can predispose to severe or recurrent gastrointestinal infections.
Hirschsprung's Disease: A congenital condition affecting the colon, leading to severe constipation or intestinal obstruction, which can sometimes present with vomiting and diarrhea in the neonatal period.