Inflammatory bowel disease (IBD) is a group of chronic conditions in which the immune system attacks the digestive tract, causing ongoing inflammation that damages the intestinal lining. An estimated 2.4 to 3.1 million Americans live with IBD, and that number is rising. The two main forms are Crohn’s disease and ulcerative colitis, which differ in where and how deeply they affect the gut.
Crohn’s Disease vs. Ulcerative Colitis
Though they share many symptoms, these two conditions behave differently inside the body. Ulcerative colitis is limited to the colon (large intestine). It typically starts in the rectum and spreads upward in one continuous stretch of inflammation, affecting only the innermost lining of the intestinal wall.
Crohn’s disease can strike anywhere from the mouth to the anus, though it most commonly affects the end of the small intestine and the beginning of the colon. Unlike ulcerative colitis, Crohn’s often skips areas, leaving patches of healthy tissue between inflamed sections. It also burrows deeper, potentially inflaming all layers of the intestinal wall. This deeper involvement is why Crohn’s is more likely to cause complications like narrowing of the intestine, fistulas (abnormal tunnels between organs), and abscesses.
What Causes IBD
The exact cause isn’t fully understood, but IBD develops when the immune system loses its ability to tolerate the normal bacteria living in the gut. In a healthy digestive tract, the immune system coexists peacefully with trillions of gut microbes. In IBD, that truce breaks down. Immune cells begin treating harmless gut bacteria as threats, launching a sustained inflammatory attack that damages the intestinal wall.
This process requires a genetic foundation. The first gene linked to Crohn’s susceptibility, called NOD2, was identified in 2001, and researchers have since mapped 163 genetic risk locations associated with IBD, 28 of which overlap between Crohn’s and ulcerative colitis. But genes alone don’t cause IBD. Environmental triggers push a genetically susceptible person over the edge.
One leading theory, sometimes called the “microflora hypothesis,” suggests that modern Western diets, widespread antibiotic use, and improved sanitation have altered the composition of gut bacteria in ways that disrupt the immune system’s balance. When the mix of protective and potentially harmful bacteria in the gut shifts too far, it creates a state of imbalance (dysbiosis) that can trigger chronic inflammation in people who are already genetically predisposed.
Common Symptoms
The hallmark symptoms of IBD are abdominal pain, persistent diarrhea, and unintended weight loss. Fatigue, fever, and a constant urgent feeling of needing to use the bathroom (even when you just went) are also common. Bloody stool is more typical of ulcerative colitis, while Crohn’s patients may experience pain focused in the lower right abdomen, where the small and large intestines connect.
IBD is a whole-body condition, not just a gut problem. About 40% of people with IBD develop joint issues, making musculoskeletal problems the most common effect outside the digestive tract. These range from peripheral joint pain in the knees or ankles to ankylosing spondylitis, a form of inflammatory arthritis affecting the spine that occurs in 5% to 10% of IBD patients. Up to 15% develop skin conditions, including painful red nodules on the shins or ulcerating sores that can appear anywhere on the body. Between 2% and 5% experience eye inflammation, which can cause redness, pain, light sensitivity, and blurred vision. Eye symptoms sometimes appear before a person even receives their IBD diagnosis.
Who Gets IBD and When
IBD follows a bimodal pattern, meaning it has two peak windows of diagnosis. The first and largest wave hits between ages 30 and 40. A second, smaller peak occurs between 60 and 70. Globally, about 6.8 million people are affected. While IBD was historically concentrated in North America and Europe, rates are climbing rapidly in newly industrialized countries across Asia, South America, and the Middle East, further supporting the link between Western lifestyle factors and disease risk.
How IBD Is Diagnosed
There’s no single test that confirms IBD. Diagnosis typically involves a combination of blood work, stool tests, and direct visualization of the intestine through endoscopy.
One of the most useful screening tools is a stool test that measures a protein called calprotectin. When the gut is inflamed, immune cells release this protein in large quantities, and it shows up reliably in stool samples. This test is particularly good at distinguishing IBD from irritable bowel syndrome (IBS), which can cause similar symptoms but doesn’t involve visible inflammation. A normal calprotectin level makes IBD unlikely and can spare you from more invasive testing.
If calprotectin or other markers suggest inflammation, the next step is endoscopy, where a doctor uses a flexible camera to examine the intestinal lining directly and take small tissue samples. Endoscopy with tissue biopsy remains the gold standard for confirming IBD, determining which type you have, and assessing how severe the inflammation is. It’s also used over time to check whether treatment is working, since the goal of modern IBD therapy is to achieve mucosal healing, meaning visible inflammation has resolved.
Treatment Approaches
IBD treatment has changed dramatically over the past two decades. The traditional approach relied on steroids to control flares, anti-inflammatory drugs called aminosalicylates for milder disease, and immune-suppressing medications for more stubborn cases. These options still play a role, but the real shift came with biologic therapies.
Biologics are lab-made proteins that target specific parts of the immune system driving inflammation. The first, approved in 1998, blocked a key inflammatory signaling molecule called TNF-alpha, which plays a central role in amplifying gut inflammation by triggering a cascade of other inflammatory signals, promoting immune cell recruitment, and damaging intestinal tissue. Since then, newer biologics have been developed that target different pathways, including one that blocks signals directing immune cells to travel to the gut in the first place, and another that interrupts a signaling pair involved in driving the specific type of immune response seen in Crohn’s disease.
The newest class of IBD medications are small-molecule drugs called JAK inhibitors, taken as pills rather than injections or infusions. These work by blocking enzymes inside immune cells that relay inflammatory signals. Some are designed to act mainly in the gut itself, minimizing effects on the rest of the body. The growing number of treatment options means that if one approach stops working or causes side effects, there are alternatives to try.
Diet and Lifestyle Management
Diet doesn’t cause IBD, but what you eat can significantly influence symptoms and inflammation. The Crohn’s Disease Exclusion Diet (CDED) is one of the most studied dietary approaches. It’s a whole-food diet designed to remove ingredients thought to harm the gut lining, disrupt gut bacteria, or trigger immune responses. The diet unfolds in three phases: a strict first phase lasting six weeks that eliminates potential triggers while emphasizing high-quality proteins and foods that support a healthy microbiome, a second phase from weeks 6 to 12 that gradually reintroduces restricted foods, and a long-term maintenance phase starting at week 13 that allows for more personalization. Clinical trials have shown the CDED can induce remission in both children and adults with Crohn’s disease.
For people whose IBD overlaps with irritable bowel symptoms like bloating and gas, a low-FODMAP approach can help. This involves temporarily reducing foods high in certain fermentable sugars, such as apples, garlic, and onions, then systematically reintroducing them to identify personal triggers. Working with a dietitian experienced in IBD is the most effective way to navigate these protocols without unnecessarily restricting your diet.
Long-Term Outlook and Surgery
IBD is a lifelong condition with periods of remission and flares. With modern treatments, many people achieve sustained remission and live full, active lives. But the disease can be progressive, particularly Crohn’s disease. Population-based data shows that about 16% of people with Crohn’s need surgery within the first year of diagnosis, a figure that rises to roughly 33% by five years and nearly 47% by ten years. Surgery for Crohn’s typically involves removing a damaged section of intestine, though it doesn’t cure the disease and inflammation can recur.
Ulcerative colitis has lower overall surgery rates, and when surgery is performed, removing the colon can effectively eliminate the disease. However, some patients who undergo this procedure develop inflammation in the surgically created pouch that replaces the colon. The CDED has shown early promise for managing this complication as well, with a small study finding that two-thirds of adults with pouch inflammation achieved remission by six weeks on the diet.