Infectious colitis is inflammation of the large intestine (colon) caused by a bacterial, viral, or parasitic infection. It typically produces diarrhea, abdominal cramping, and sometimes fever or bloody stools. Most cases resolve on their own with fluids and rest, but certain infections require specific treatment, and a small percentage can become serious.
How Infections Inflame the Colon
When a pathogen reaches the colon, it invades the lining and triggers an immune response. White blood cells flood the tissue, creating swelling, redness, and sometimes ulceration. The inflammation can be patchy or spread across the entire colon, depending on the organism involved.
Different pathogens cause damage in different ways. Campylobacter, one of the most common culprits, invades the surface layer of the intestinal wall and produces a swollen, bloody lining. A dangerous strain of E. coli (O157:H7) causes hemorrhagic colitis by damaging tiny blood vessels in the colon wall, forming small clots inside capillaries and cutting off blood flow to the tissue. Parasites like Entamoeba histolytica use extensions of their cell body to burrow into the colon lining, creating deep ulcers that bleed.
Common Causes
Bacteria
Campylobacter jejuni is the single most frequent bacterial cause in the United States, isolated roughly twice as often as Salmonella and seven times more often than Shigella in patients with diarrhea. Altogether, bacterial infections account for 4% to 11% of all diarrheal illness in the U.S. Other common bacterial causes include E. coli, Yersinia, and Clostridioides difficile (C. diff), which often develops after antibiotic use disrupts the normal balance of gut bacteria.
Viruses
Norovirus and rotavirus are the most common viral causes. Viral colitis tends to be milder and shorter-lived than bacterial infection, though it spreads extremely easily through contaminated food, water, or surfaces.
Parasites
Entamoeba histolytica is the most significant parasitic cause and is found worldwide, with higher rates in Central and South America, Africa, and India. It spreads through the fecal-oral route, often via contaminated water. Infection can remain silent in many people, but when it becomes invasive it causes abdominal pain, fever, and bloody diarrhea with mucus. Other parasitic causes include Cryptosporidium, Cyclospora, and Balantidium coli, which can produce symptoms nearly identical to amoebic colitis.
How It Spreads
Most infectious colitis is acquired through contaminated food or water. Undercooked poultry is a classic source of Campylobacter and Salmonella. Ground beef is the primary vehicle for E. coli O157:H7. Produce washed with contaminated water can carry parasites or bacteria. Person-to-person spread is common with norovirus and C. diff, especially in hospitals, nursing homes, and daycare settings.
People at higher risk for severe disease include young children, older adults, anyone with a weakened immune system, and people taking antibiotics or acid-reducing medications. Recent hospitalization is a major risk factor for C. diff specifically.
Symptoms to Recognize
The hallmark symptom is diarrhea, which ranges from watery to frankly bloody depending on the pathogen. Most people also experience abdominal cramping that worsens before a bowel movement and eases somewhat afterward. Fever is common with bacterial and parasitic infections but less reliable with viral causes.
Other symptoms include:
- Urgency and tenesmus: a painful feeling that you need to have a bowel movement even when the rectum is empty
- Nausea or vomiting: more prominent with viral infections and food poisoning
- Mucus or blood in stool: suggests an invasive organism like Shigella, Campylobacter, E. coli O157:H7, or Entamoeba
- Dehydration signs: dry mouth, dark urine, dizziness, and fatigue, particularly in young children and older adults
Symptoms typically begin one to three days after exposure and last anywhere from a few days to two weeks. Bloody diarrhea, high fever, or signs of dehydration signal a more serious infection that needs medical evaluation.
How It’s Diagnosed
Stool testing is the cornerstone of diagnosis. Traditional stool cultures grow the organism in a lab, which takes 48 to 96 hours and remains one of the most sensitive methods available. The downside is speed, and cultures can sometimes detect harmless strains that aren’t actually causing disease.
Newer molecular tests (PCR-based panels) can identify multiple pathogens from a single stool sample on the same day. These are highly sensitive and specific, making them increasingly the first test ordered. For C. diff in particular, labs often use a two-step approach: a rapid antigen screen that takes under an hour, followed by a molecular or toxin test to confirm whether the organism is actually producing the toxins responsible for disease.
In some cases, a colonoscopy or flexible sigmoidoscopy may be needed, especially if inflammatory bowel disease is a concern or the diagnosis remains unclear after stool testing. Imaging like a CT scan can help identify complications such as severe inflammation or colon dilation.
Treatment and Recovery
Most cases of infectious colitis are self-limiting, meaning the infection clears without antibiotics. The priority is staying hydrated. Oral rehydration solutions work well for mild to moderate dehydration. Small, frequent sips are easier to tolerate than large amounts at once.
Antibiotics are reserved for specific situations: infections caused by certain bacteria like Shigella, severe Campylobacter infections, parasitic causes like Entamoeba, and C. diff. Importantly, antibiotics can make some infections worse. E. coli O157:H7 should not be treated with antibiotics because killing these bacteria can trigger them to release more toxin, raising the risk of a dangerous kidney complication.
For C. diff, treatment guidelines have shifted in recent years. Targeted oral antibiotics that act mainly inside the gut are now first-line therapy for initial episodes. A standard course runs 10 days, and most people improve within the first few days of treatment. Recurrent C. diff infections, which affect roughly 1 in 5 patients after a first episode, may require extended or pulsed courses and sometimes newer therapies that restore healthy gut bacteria.
Most people recover fully within one to two weeks. Returning to a normal diet gradually, starting with bland foods, helps the colon heal. Probiotic-rich foods or supplements may help restore gut flora after antibiotic-associated cases, though evidence varies by strain and situation.
Potential Complications
Serious complications are uncommon but important to recognize. Severe dehydration is the most frequent, especially in children and elderly adults. E. coli O157:H7 can trigger hemolytic uremic syndrome, a condition that damages red blood cells and the kidneys, occurring in about 5% to 10% of infections with that strain.
Toxic megacolon is the most feared complication. The colon stops contracting normally and dilates beyond 5.5 centimeters, trapping gas and stool. It produces systemic signs of toxicity: high fever, rapid heart rate, low blood pressure, and confusion. The mortality rate ranges from 38% to 80% when perforation, sepsis, or organ failure develop. This complication is more commonly associated with C. diff and inflammatory bowel disease than with typical foodborne pathogens, but any severe colitis can theoretically progress to it.
Reactive arthritis is another recognized complication, occurring weeks after infection with Campylobacter, Salmonella, Shigella, or Yersinia. It causes joint pain and swelling, typically in the knees and ankles, and resolves over weeks to months in most cases.