Blood is not a normally present in the stool and usually indicates a underlying, serious medical condition. Blood in the stool can either be visibly seen or gross blood, or cannot be seen or occult blood. The presence of blood in the stool needs to be examined by your physician.

Colon cancer

According to the American College of Gastroenterology, colorectal cancer is the second most common cancer killer overall and third most common cause of cancer-related death in the United States in both males and females. This is where malignant or cancerous cells accumulate in the colon or rectum. Testing for fecal occult blood is a common, simple way to screen for colon cancer.

Hemorrhoids

Hemorrhoids are a very common condition in which the veins in the anus become swollen. Bright red gross blood found on the stool is a common symptom. The condition is also known as "piles," and up to half of people 50 years of age or older have hemorrhoids.

Diverticulosis

This gastrointestinal condition is also most frequently seen in the older population (older than 60) is characterized by small sacs that develop on the large intestine.

Ulcers

Ulcers are a very common result of bleeding and can result with blood in the stool. Two common causes of ulcers is the bacterium helicobacter pylori and the use of aspirin.

Infections

Certain types of infections can result in bloody stool. This is also known as dysentery. Certain bacteria like shigella and parasites like entamoeba histolytica have the ability to penetrate and damage the intestinal lining causing bleeding.

Source:

The American College of Gastroenterology-Colon cancer

American Gastroenterological Association-Digestive conditions