Gastroenteritis: This is the most common cause of loose bowel movements in infants, often due to viral infections. The symptoms can range from mild to severe and are usually self-limiting.

Lactose Intolerance: Infants may have difficulty digesting lactose, a sugar found in milk, leading to loose bowel movements. This condition can be primary (due to lactase deficiency) or secondary (following a gastrointestinal infection).
Formula Intolerance: Some infants may react to certain components in their formula, leading to gastrointestinal symptoms including loose stools.
Infectious Diarrhea (Bacterial): Bacterial infections can cause diarrhea in infants, though they are less common than viral causes.

Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially cutting off blood flow. It's a medical emergency and can present with intermittent abdominal pain, vomiting, and bloody stools, in addition to loose bowel movements.
Hirschsprung's Disease: A congenital condition characterized by the absence of nerve cells in parts of the bowel, leading to severe constipation or, in some cases, diarrhea due to enterocolitis.
Inflammatory Bowel Disease (IBD): Though rare in infants, conditions like Crohn's disease and ulcerative colitis can present with chronic diarrhea, among other symptoms.

Celiac Disease: An autoimmune reaction to gluten, leading to inflammation and damage in the small intestine. It's rare in infants but can cause chronic diarrhea.
Congenital Infections: Certain infections acquired during pregnancy, such as toxoplasmosis or cytomegalovirus, can lead to gastrointestinal symptoms in newborns, including diarrhea.
Immunodeficiency: Infants with immunodeficiency disorders may be more prone to infections, including those causing gastrointestinal symptoms like diarrhea.