Bloody diarrhea has a wide range of causes, from bacterial infections that resolve on their own to chronic conditions like inflammatory bowel disease and, less commonly, reduced blood flow to the colon. The color and amount of blood, whether you’re experiencing pain, and how quickly symptoms developed all point toward different explanations. Here’s what can cause it and how to tell the difference.
Bacterial Infections
Infections are the most common cause of bloody diarrhea in otherwise healthy people. Several types of bacteria can invade or damage the lining of the colon enough to produce visible blood in stool.
Campylobacter is one of the most frequent culprits. It has an incubation period of 2 to 5 days after exposure, typically from undercooked poultry or contaminated water. Diarrhea usually starts within 24 hours of the first abdominal cramps and can range from watery to grossly bloody.
Shigella causes dysentery, a particularly aggressive form of bloody diarrhea often accompanied by high fever, severe cramping, and mucus in the stool. It spreads easily from person to person, especially in daycare settings and areas with limited sanitation.
Shiga toxin-producing E. coli (STEC), including the well-known O157:H7 strain, deserves special attention. It produces toxins that damage blood vessels in the intestinal wall, causing bloody diarrhea that often starts without fever. STEC infections can progress to a serious complication affecting the kidneys, particularly in young children and older adults. Antibiotics are generally avoided for STEC because they may worsen the risk of kidney damage.
Salmonella more commonly causes watery diarrhea, but certain strains can produce blood in stool, especially in people with weakened immune systems.
Most bacterial infections are picked up through contaminated food or water. Symptoms typically develop within one to five days of exposure and resolve within a week, though some infections take longer.
Inflammatory Bowel Disease
When bloody diarrhea keeps coming back over weeks or months rather than appearing suddenly after a meal, inflammatory bowel disease (IBD) becomes a leading suspect. The two main forms, ulcerative colitis and Crohn’s disease, affect the digestive tract differently and produce distinct patterns of bleeding.
Ulcerative colitis is strongly associated with bloody diarrhea. It causes inflammation limited to the colon, always starting at the rectum. Typical symptoms include an urgent need to use the bathroom, the feeling that you still need to go after a bowel movement, and cramping centered in the lower abdomen. Blood is almost always visible in the stool.
Crohn’s disease, by contrast, often involves the small intestine and produces diarrhea that is frequently nonbloody, along with belly pain and unintended weight loss. When Crohn’s does affect the colon, however, it can cause bleeding as well. The distinction matters because the two conditions are treated differently, and a gastroenterologist uses the pattern of symptoms, along with imaging and biopsies, to tell them apart.
Ischemic Colitis
Ischemic colitis happens when blood flow to part of the colon drops enough to injure the tissue. It causes sudden cramping on the left side of the abdomen, followed by an urgent need to have a bowel movement and bright red or maroon blood in the stool. The condition mostly affects adults older than 60, and it’s more common in women.
Risk factors include atherosclerosis (fatty buildup in the arteries), heart failure, low blood pressure, prior abdominal surgery, and clotting disorders like factor V Leiden or sickle cell disease. Cocaine and methamphetamine use can trigger it in younger adults by constricting blood vessels. Even heavy exercise like marathon running can reduce blood flow to the colon enough to cause transient ischemia. Most mild cases heal on their own with supportive care, but severe episodes can require hospitalization.
Diverticular Bleeding
Diverticular bleeding is a common cause of significant rectal bleeding in older adults. Small pouches called diverticula form in the colon wall over time, and a blood vessel near one of these pouches can rupture. The hallmark is painless, sudden-onset bleeding that can be heavy enough to be alarming. The blood is often dark red or maroon.
This is distinct from diverticulitis, which involves inflammation or infection of the pouches and typically causes left-sided abdominal pain, fever, and changes in bowel habits, but rarely heavy bleeding. Research from the REMAD registry found that diverticulitis and diverticular bleeding are largely unrelated conditions that rarely occur together. Diverticulitis tends to affect younger patients, while diverticular bleeding is more common with advancing age. Painless diverticular bleeding warrants a trip to the emergency department because these bleeds can be severe.
Medications That Damage the Colon
Nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen and naproxen are well-known for causing stomach ulcers, but they can also damage the colon. NSAIDs work by blocking enzymes involved in producing prostaglandins, protective compounds that help maintain the mucous lining of the gut. Without that protection, the tissue becomes vulnerable to ulceration and bleeding. Long-term or heavy NSAID use can lead to colonic ulcers that produce bloody diarrhea.
Blood thinners don’t directly damage the colon, but they can unmask bleeding from existing conditions like diverticula or small polyps that might otherwise go unnoticed. If you’re on a blood thinner and notice blood in your stool, that combination needs prompt medical attention.
Causes in Children
In infants and toddlers, bloody diarrhea raises a different set of concerns. Bacterial infections are still a possibility, but one condition unique to young children is intussusception, where one segment of the intestine telescopes into the adjacent section. This blocks normal flow and damages the tissue, producing a characteristic “currant jelly” stool: a mix of blood and mucus with a dark, jelly-like appearance. Intussusception also causes episodes of intense cramping, vomiting, and drawing the knees to the chest. It’s a medical emergency that usually requires treatment within hours.
Cow’s milk protein allergy is another cause of bloody stools in infants, though the blood is typically streaky rather than profuse. It often appears in the first few months of life and resolves when the offending protein is removed from the diet.
How Doctors Identify the Cause
When you show up with bloody diarrhea, the first priority is figuring out whether the cause is infectious, inflammatory, or structural. If infection is suspected, the standard approach is a gastrointestinal pathogen panel, a single stool test that screens for Salmonella, Shigella, Campylobacter, Yersinia, C. difficile, and Shiga toxin-producing E. coli all at once. These molecular tests (PCR-based panels) have largely replaced older culture methods because they’re more sensitive and return results faster.
If infection is ruled out or if symptoms are chronic or recurring, a colonoscopy is typically the next step. This allows direct visualization of the colon lining and the ability to take tissue samples. The pattern of inflammation seen on biopsy helps distinguish ulcerative colitis from Crohn’s disease, ischemic colitis, and other causes.
Signs That Need Immediate Attention
Not every case of bloody diarrhea requires an emergency visit, but certain warning signs do. A heart rate above 100 beats per minute, systolic blood pressure below 115, pale skin, dizziness when standing, or feeling faint all suggest significant blood loss. Large-volume bleeding that fills the toilet bowl, especially if it’s painless and sudden, should prompt a trip to the emergency department regardless of other symptoms.
Bloody diarrhea paired with high fever, severe abdominal tenderness, or signs of dehydration (very dark urine, dry mouth, confusion) also warrants urgent evaluation. In young children, the combination of bloody stool, vomiting, and inconsolable crying should be treated as an emergency until proven otherwise.