Infectious Gastroenteritis: This is often the most common cause of bloody diarrhea, especially in children and young adults. It can be caused by a variety of pathogens including bacteria (e.g., Shigella, Salmonella, E. coli), viruses, and parasites. The presence of blood in the stool is usually accompanied by other symptoms such as fever, abdominal cramps, and vomiting.

Inflammatory Bowel Disease (IBD): Conditions like Ulcerative Colitis and Crohn's Disease can cause chronic bloody diarrhea due to inflammation and ulceration of the bowel lining. Other symptoms may include weight loss, abdominal pain, and urgency.
Food Poisoning: Certain types of food poisoning, such as that caused by E. coli or Salmonella, can lead to bloody diarrhea. This is often associated with the consumption of contaminated food or water.
Diverticulitis: Inflammation of the diverticula in the colon can cause bloody diarrhea, especially in older adults. Pain, usually on the lower left side of the abdomen, is a common accompanying symptom.

Intussusception: Although more common in children, intussusception can occur in adults and is a medical emergency. It involves the telescoping of one portion of the intestine into another, which can cause bloody diarrhea, severe abdominal pain, and vomiting.
Ischemic Colitis: Reduced blood flow to the colon can cause bloody diarrhea, abdominal pain, and potentially lead to gangrene if not promptly treated. This condition is more common in older adults and those with cardiovascular disease.
Colon Cancer: While less common as a cause of acute bloody diarrhea, colon cancer can present with bloody stools, especially if the tumor is located in the distal colon. Other symptoms may include changes in bowel habits, weight loss, and iron deficiency anemia.

Hemolytic Uremic Syndrome (HUS): This condition, often associated with E. coli infections, can cause bloody diarrhea followed by acute kidney failure and hemolytic anemia. It's more common in children but can occur in adults.
Angiodysplasia: Vascular malformations in the colon can cause recurrent episodes of bloody diarrhea, especially in older adults. This condition may not present with other symptoms besides the bleeding.
Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause painless bloody diarrhea, especially in children and young adults, due to the presence of ectopic gastric mucosa that can ulcerate and bleed.