Viral Gastroenteritis: This is the most common cause of diarrhea in children and adolescents, often due to viruses like norovirus or rotavirus, and typically presents without nausea or vomiting in some cases.

Bacterial Gastroenteritis: Caused by bacteria such as E. coli, Salmonella, or Campylobacter, which can lead to diarrhea. The absence of nausea or vomiting does not rule out this diagnosis.
Food Intolerance or Allergy: Conditions like lactose intolerance or a food allergy could cause diarrhea without nausea or vomiting.
Irritable Bowel Syndrome (IBS): A chronic condition that can cause diarrhea, among other symptoms, without an infectious cause.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can present with chronic diarrhea and are crucial to diagnose early to prevent complications.
Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause intermittent diarrhea and is a medical emergency.
Appendicitis: Although classically presents with abdominal pain, nausea, and vomiting, atypical presentations can occur, especially in children, and must be considered.

Celiac Disease: An autoimmune disorder that causes the immune system to react to gluten, leading to small intestine damage and diarrhea.
Giardiasis: An intestinal infection caused by the parasite Giardia lamblia, which can lead to chronic diarrhea if not treated.
Microscopic Colitis: A condition characterized by chronic, watery diarrhea, often without an identifiable cause, and diagnosed through colonoscopy and biopsy.