Viral Gastroenteritis: This is the most common cause of diarrhea in infants, often accompanied by diaper rash due to the frequent and loose stools. Viruses such as rotavirus, norovirus, and adenovirus are common culprits.

Bacterial Gastroenteritis: Though less common than viral causes, bacteria like E. coli, Salmonella, and Shigella can cause diarrhea in infants. The presence of blood in the stool or a high fever might suggest a bacterial etiology.
Lactose Intolerance: Temporary lactose intolerance can occur after a viral gastroenteritis, leading to continued diarrhea in infants fed with lactose-containing formulas.
Food Allergy or Intolerance: Allergies to proteins in formula (e.g., cow's milk protein allergy) or introduced foods can cause diarrhea and skin rashes, including diaper rash.

Intussusception: Although rare, intussusception is a condition where a part of the intestine slides into an adjacent part, potentially causing bowel obstruction. It's a surgical emergency and can present with diarrhea, abdominal pain, and sometimes currant jelly stool.
Infectious Diarrhea with Systemic Involvement (e.g., Urinary Tract Infection): In young infants, diarrhea can sometimes be a presenting symptom of a urinary tract infection, especially if associated with fever and other systemic signs.
Hirschsprung's Disease: A congenital condition characterized by the absence of ganglion cells in the distal bowel, leading to severe constipation, but can occasionally present with diarrhea in the early stages due to enterocolitis.

Congenital Infections (e.g., Congenital CMV): Certain congenital infections can cause chronic diarrhea among other symptoms.
Immunodeficiency: Conditions affecting the immune system can predispose infants to severe, recurrent, or persistent diarrhea due to opportunistic infections.
Celiac Disease: An autoimmune reaction to gluten, leading to small intestine damage and malabsorption. It's rare in infants as young as 6 months but can occur if gluten has been introduced into the diet.