CDI stands for Clostridioides difficile infection, a bacterial infection of the colon that causes watery diarrhea, abdominal pain, and fever. It’s one of the most common healthcare-associated infections, and it happens when a bacterium called C. difficile (often shortened to C. diff) overgrows in the large intestine, usually after antibiotics have wiped out the normal gut bacteria that keep it in check. About 500,000 cases occur in the United States each year, and the infection can range from mild diarrhea to life-threatening colon inflammation.
How C. Diff Damages the Gut
C. difficile bacteria produce two toxins that do the actual damage. These toxins latch onto cells lining the intestinal wall, get absorbed inside, and then disable proteins that hold those cells together. The result is a breakdown of the barrier that normally keeps the contents of your intestines contained. Gaps open between cells, fluid leaks into the colon, and diarrhea follows.
At the same time, the toxins trigger a powerful inflammatory response. Your immune system floods the area with white blood cells, particularly neutrophils, which cause further tissue damage. In severe cases, this inflammation produces a characteristic pattern called pseudomembranous colitis, where yellowish-white plaques form on the colon lining. That combination of barrier breakdown and intense inflammation is what makes CDI so much more than a simple stomach bug.
Who Gets CDI and Why
The single biggest risk factor is recent antibiotic use. Antibiotics kill off the diverse community of bacteria in your gut, creating an opening for C. diff to multiply unchecked. The highest-risk antibiotics include fluoroquinolones, clindamycin, broad-spectrum penicillins, and broad-spectrum cephalosporins. You don’t have to be taking these drugs for weeks; even a short course can shift the balance enough.
Other factors raise the risk significantly. People over 65 are more vulnerable, as are those with weakened immune systems, multiple chronic health conditions, or a history of recent hospitalization. The hospital connection matters because C. diff spores survive on surfaces for months and spread easily in healthcare settings. Acid-reducing medications like proton pump inhibitors have been investigated as a possible risk factor, though the evidence remains mixed, with some studies finding no statistically significant link.
Symptoms to Recognize
The hallmark of CDI is watery diarrhea, typically three or more loose stools per day, often with a distinctive foul smell. Crampy abdominal pain and tenderness are common, especially in the lower abdomen. Mild cases may involve just frequent loose stools, while moderate cases add low-grade fever and more persistent pain.
Severe CDI looks different. A white blood cell count of 15,000 or higher, signs of kidney stress, or significant dehydration signal that the infection is escalating. At the extreme end, called fulminant CDI, the colon can stop moving entirely (a condition called ileus), dangerously dilate (toxic megacolon), or cause a drop in blood pressure. Fulminant cases are medical emergencies and sometimes require surgery to remove part of the colon.
How CDI Is Diagnosed
Doctors test for CDI using a stool sample, but no single test is perfect, so many labs use a two-step approach. The first step is usually a rapid screening test that detects a protein produced by C. diff bacteria. Results come back in under an hour, but a positive result only confirms the bacterium is present, not that it’s actively producing toxins and causing disease.
If the screen is positive, a second test checks specifically for the toxins that cause symptoms. This toxin test is cheap and fast but not highly sensitive, meaning it can miss some true infections. Alternatively, some labs use a molecular test (PCR) that detects the gene responsible for toxin production. PCR is extremely sensitive but has a drawback: it can come back positive in people who carry the bacteria without being sick. That’s why testing is only recommended in patients who actually have diarrhea. Testing people without symptoms leads to false diagnoses and unnecessary treatment.
Treatment for a First Episode
CDI is treated with specific antibiotics that target the C. diff bacteria while sparing other gut flora as much as possible. Current guidelines from the Infectious Diseases Society of America favor fidaxomicin as the preferred first-line option over vancomycin, based on its lower recurrence rates. Vancomycin remains a fully acceptable alternative and is widely used.
A standard course of either antibiotic lasts 10 days. Both are taken by mouth, which is important because the drugs need to reach the colon directly. Most people start feeling better within a few days, though completing the full course is essential. Stopping early increases the chance of the infection coming back.
The Problem of Recurrence
One of the most frustrating aspects of CDI is how often it returns. Roughly 20% of patients who recover from an initial episode will have a recurrence, typically within two to eight weeks. After a second episode, the odds climb even higher, with recurrence rates reaching up to 60% in patients who’ve already relapsed once. Each recurrence makes the next one more likely, creating a cycle that antibiotics alone struggle to break.
Recurrence happens because C. diff produces spores, dormant forms of the bacterium that antibiotics can’t kill. These spores sit quietly in the colon and germinate once the antibiotic course ends, reigniting the infection before normal gut bacteria have had time to recover. Risk factors for recurrence include age over 65, ongoing antibiotic use for other conditions, and having inflammatory bowel disease.
For recurrent episodes, treatment strategies shift. Fidaxomicin given in an extended-pulsed regimen (full doses for five days followed by every-other-day dosing for about three weeks) helps the gut microbiome recover between doses. Vancomycin can also be given in a tapered and pulsed schedule for the same reason.
Fecal Microbiota Transplant for Stubborn Cases
When antibiotics fail to break the cycle of recurrence, fecal microbiota transplant (FMT) offers a fundamentally different approach. Instead of trying to kill C. diff, FMT restores the diverse community of gut bacteria that keeps it suppressed naturally. The procedure involves introducing processed stool from a healthy, screened donor into the patient’s colon.
The results are striking. A single FMT procedure resolves recurrent CDI in roughly 93% of patients, and with repeated procedures the success rate climbs to nearly 98%. The FDA has approved standardized microbiota-based therapies derived from donor stool, making access more consistent than the early days of FMT when individual donors were needed for each procedure.
Why Hand Sanitizer Doesn’t Work
C. diff spores are uniquely resistant to alcohol-based hand sanitizers. The spores have a tough outer coating that alcohol simply can’t penetrate, which is why the CDC specifically recommends washing hands with soap and water to prevent person-to-person spread. The physical friction of handwashing is what removes spores from skin.
For surfaces, standard hospital disinfectants may not be enough either. Chlorine bleach is effective at killing C. diff spores and is the recommended cleaning agent for contaminated areas, both in healthcare facilities and at home. If you’re caring for someone with CDI, cleaning bathrooms and high-touch surfaces with a bleach-based product is one of the most practical steps you can take to limit transmission.
Community-Acquired vs. Hospital-Acquired CDI
CDI was long considered strictly a hospital problem, but a growing proportion of cases now begin in the community. People who haven’t been hospitalized or taken antibiotics recently can still develop CDI, though it’s less common. Community-acquired cases tend to affect younger, healthier people and are often milder, but they can still recur and occasionally become severe. The shift matters because it means CDI should be on the radar even for people who haven’t set foot in a hospital recently, particularly if unexplained watery diarrhea persists for more than a day or two.