The occurrence of chronic diarrhea in infants younger than three months suggests disaccharidase deficiency, cow's milk or soy protein intolerance, cystic fibrosis or an immunodeficiency state, while chronic diarrhea in children three to 18 years of age suggests celiac disease, late-onset primary lactose deficiency and inflammatory bowel disease. Gastrointestinal infection is the most common cause of chronic diarrhea in children of all ages. Diarrhea that develops after the introduction of cow's milk, cereals and fruits suggests an enzyme deficiency or protein intolerance. Watery, explosive stools suggest sugar intolerance, and foul-smelling, greasy, bulky stools suggest fat malabsorption. Marked weight loss suggests malabsorption, inflammatory bowel disease, hyperthyroidism or malignancy. The presence of neutrophils or red blood cells in the stool indicates bacterial gastroenteritis or inflammatory bowel disease, while the presence of eosinophils suggests protein intolerance or parasitic infestation. A toddler who is thriving and cheerful despite having diarrhea may have chronic nonspecific diarrhea of childhood.