IBS is not an active infection. It is a chronic functional disorder, meaning the gut behaves abnormally without an ongoing pathogen causing the problem. However, the relationship between IBS and infection is more nuanced than a simple “no.” A significant subset of IBS cases begin directly after a bout of food poisoning or stomach flu, and bacterial overgrowth in the small intestine is found in more than a third of IBS patients. So while IBS itself is not something you can catch or cure with antibiotics the way you would a gut infection, infections play a real role in how and why IBS develops.
How IBS Differs From a Gut Infection
A gastrointestinal infection is caused by a specific pathogen (bacteria, virus, or parasite) that invades the gut, causes acute symptoms like vomiting and diarrhea, and typically resolves once the immune system clears the organism or antibiotics kill it. IBS, by contrast, is diagnosed based on a pattern of symptoms lasting at least six months: recurring abdominal pain at least one day per week, linked to changes in how often you have bowel movements or what they look like. There is no detectable pathogen driving those symptoms.
The standard diagnostic criteria, known as the Rome IV criteria, don’t include any infection-related markers. Doctors diagnose IBS based on symptom patterns and by ruling out other conditions like inflammatory bowel disease or celiac disease. If stool tests were to find an active bacterial or parasitic infection, that would be a different diagnosis entirely.
When an Infection Triggers IBS
One of the best-understood pathways to IBS is through a prior infection. This is called post-infectious IBS, and it develops when symptoms of abdominal pain, bloating, and diarrhea persist long after the original bug has been cleared from the body. The key detail: the pathogen is gone, but the gut doesn’t return to normal.
Research on patients recovering from Campylobacter infections (a common cause of food poisoning) has shown that intestinal biopsies still contain elevated immune cells and signs of inflammation months after the infection resolved. Specifically, patients who went on to develop post-infectious IBS had a 25% increase in certain hormone-producing cells in the gut lining compared to healthy controls or people with other forms of IBS. They also showed increased gut permeability, sometimes called “leaky gut,” where the intestinal barrier doesn’t seal properly. Even three months after the initial illness, rectal biopsies from post-infectious IBS patients showed higher levels of inflammatory signaling molecules compared to people who had the same stomach bug but recovered fully.
Bacterial infections like Campylobacter and Salmonella tend to cause a more lasting form of post-infectious IBS than viral stomach bugs, which are more likely to cause symptoms that fade over time. Post-infectious IBS also tends to be diarrhea-predominant, which makes sense given that the original trigger was an illness that disrupted normal bowel function.
What Actually Goes Wrong in the Gut
Even in IBS cases that don’t follow an obvious infection, the gut shows measurable biological changes. Studies comparing intestinal biopsies from IBS patients and healthy controls have found higher densities of mast cells and eosinophils (types of immune cells involved in inflammation and allergic responses) in the lower colon. The density of degranulating mast cells, meaning mast cells actively releasing their contents, correlates with the severity of abdominal pain. Mast cells located near gut nerves appear to be particularly important in driving that pain signal.
This is sometimes described as low-grade inflammation. It’s not the kind of dramatic, visible inflammation you’d see in Crohn’s disease or ulcerative colitis. Colonoscopies in IBS patients typically look normal to the naked eye. But at the microscopic level, the immune system is subtly more active than it should be, and the gut lining is more permeable than normal. These changes affect how the gut muscles contract, how sensitive the intestinal nerves are, and how quickly food moves through the digestive tract.
The Role of Bacterial Overgrowth
About 38% of IBS patients test positive for small intestinal bacterial overgrowth, or SIBO, a condition where bacteria that normally live in the large intestine colonize the small intestine in excessive numbers. IBS patients are nearly five times more likely to have SIBO than people without IBS. This isn’t an “infection” in the traditional sense. These aren’t invading pathogens. They’re bacteria that belong in your gut, just in the wrong place or in the wrong amounts.
IBS patients with diarrhea-predominant symptoms tend to have reduced levels of beneficial bacteria like Lactobacillus and Bifidobacterium, with higher levels of potentially problematic bacteria from the Enterobacteriaceae family. Whether this imbalance is a cause of IBS symptoms or a consequence of the altered gut environment remains an open question, but it helps explain why some IBS patients respond to treatments that target gut bacteria.
Why Antibiotics Sometimes Help
If IBS isn’t an infection, it might seem strange that some patients improve on antibiotics. One antibiotic in particular, rifaximin, is approved for diarrhea-predominant IBS and works primarily in the gut without being absorbed into the bloodstream. It was originally thought to work by correcting bacterial overgrowth, and it does help some SIBO patients. But research suggests its benefits in IBS may go beyond simply killing excess bacteria. Preclinical studies indicate rifaximin may also reduce inflammatory signaling and improve gut barrier function, addressing some of the underlying changes that keep IBS symptoms going.
That said, antibiotics don’t work for all IBS patients, and the improvement is often modest. This fits with the understanding that IBS has multiple overlapping causes. For some people, bacterial overgrowth is the main driver. For others, it’s nerve sensitivity, immune activation, disrupted gut motility, or the lingering aftermath of a past infection. These mechanisms can overlap, which is why IBS is notoriously difficult to treat with any single approach.
The Bottom Line on Infection and IBS
IBS is not an active infection, and you cannot transmit it to someone else. But infections are one of the clearest and most well-documented triggers for developing IBS. What happens is that the initial infection sets off a chain of changes in gut permeability, immune activity, nerve sensitivity, and bacterial balance that persist long after the original pathogen is gone. For people whose IBS started after food poisoning or a severe stomach bug, the connection to infection is direct, even though the infection itself is over. For others, similar gut changes develop through different pathways, including stress, diet, and shifts in the microbiome that have nothing to do with catching a bug.