Dozens of diseases can affect the digestive system, ranging from brief bouts of stomach upset to chronic conditions that require lifelong management. Roughly 20% of the U.S. population experiences acid reflux symptoms at least once a week, and tens of millions live with conditions like irritable bowel syndrome, inflammatory bowel disease, or gallstones. Here’s a closer look at the most common ones, how they develop, and what they feel like.
Acid Reflux and GERD
Gastroesophageal reflux disease, or GERD, is one of the most widespread digestive conditions. It happens when the barrier between your stomach and esophagus doesn’t close properly, allowing stomach acid to wash back up into the esophagus. That barrier is a high-pressure zone created by a ring of muscle at the bottom of the esophagus working together with the diaphragm. When either component weakens, or when a hiatal hernia shifts their alignment, acid escapes more easily.
After you eat, a layer of acidic gastric juice sits on top of the food in your stomach, right near the junction with the esophagus. In people with GERD, this acid layer extends higher than it should, making reflux more likely. The body normally clears acid from the esophagus through swallowing and saliva, but if that process is slow or the reflux is frequent, the lining of the esophagus takes damage over time. Chronic, untreated GERD can lead to Barrett’s esophagus, a condition where the esophageal lining changes to resemble intestinal tissue, which raises the risk of esophageal cancer.
Common symptoms include heartburn, regurgitation of food or sour liquid, difficulty swallowing, and a chronic cough. Many people manage mild reflux with dietary changes (avoiding large meals, acidic foods, and eating close to bedtime) and over-the-counter acid reducers.
Irritable Bowel Syndrome
IBS is a functional disorder, meaning the digestive tract looks structurally normal but doesn’t work the way it should. It affects an estimated 15 million people in the U.S. alone. The hallmark is recurrent abdominal pain occurring at least one day per week over three months, tied to changes in how often you have bowel movements or how your stool looks. To qualify as IBS, symptoms need to have started at least six months before diagnosis.
IBS is typically classified into subtypes based on whether constipation, diarrhea, or a mix of both predominates. The exact cause isn’t fully understood, but it involves a combination of heightened gut sensitivity, disrupted communication between the brain and the gut, and sometimes changes in the gut microbiome. Stress, certain foods, and hormonal shifts can all trigger flare-ups. Treatment usually focuses on dietary adjustments (a low-FODMAP diet is one common approach), stress management, and targeted medications for the most bothersome symptom.
Inflammatory Bowel Disease
Inflammatory bowel disease, or IBD, is an umbrella term for two chronic conditions: ulcerative colitis and Crohn’s disease. Unlike IBS, IBD involves visible inflammation and tissue damage that shows up on imaging and biopsies. The two forms differ in important ways.
Ulcerative Colitis
Ulcerative colitis is confined to the large intestine. It typically begins in the rectum and spreads upward in a continuous stretch with no gaps of healthy tissue in between. The inflammation stays in the innermost lining of the colon, called the mucosal layer. Symptoms include bloody diarrhea, urgency, cramping, and fatigue. Flares can alternate with periods of remission.
Crohn’s Disease
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 spots. The inflammation penetrates deeper into the intestinal wall, sometimes through all its layers. This deeper involvement can lead to complications like strictures (narrowed sections of bowel), fistulas (abnormal tunnels between organs), and abscesses.
Both forms of IBD are driven by an overactive immune response, and both carry an increased risk of colorectal cancer over time. Treatment typically involves medications that calm the immune system, and some people eventually need surgery to remove severely damaged sections of bowel.
Celiac Disease
Celiac disease is an autoimmune condition triggered by gluten, a protein found in wheat, barley, and rye. When someone with celiac disease eats gluten, their immune system attacks the lining of the small intestine, destroying the tiny finger-like projections called villi that are responsible for absorbing nutrients. Once villi are damaged, your body can’t absorb enough vitamins, minerals, and calories no matter how much you eat.
Symptoms go well beyond the gut. Diarrhea, bloating, and abdominal pain are common, but many people also develop iron-deficiency anemia, bone loss, skin rashes, fatigue, and neurological symptoms like tingling in the hands and feet. Some people have no obvious digestive symptoms at all, which is why celiac disease is often underdiagnosed. The only effective treatment is a strict, lifelong gluten-free diet. Once gluten is removed, the intestinal lining typically heals over weeks to months.
Gallstones
Gallstones form in the gallbladder, a small organ that stores bile used to digest fats. They come in several types. Cholesterol stones are the most common in Western countries and form when the bile contains too much cholesterol relative to the bile salts that normally keep it dissolved. Pigment stones are made primarily of bilirubin, a waste product from red blood cell breakdown.
The risk factors for cholesterol gallstones read like a checklist: being female, being over 40, having obesity or metabolic syndrome, losing weight rapidly, multiple pregnancies, and estrogen-based medications like birth control pills. Pregnancy is a particularly strong risk factor because high progesterone levels slow gallbladder contractions, causing bile to sit longer and become more concentrated. Prolonged fasting and very low-calorie diets raise the risk for the same reason.
Many gallstones cause no symptoms and are discovered incidentally. When a stone blocks the duct leading out of the gallbladder, it produces sudden, intense pain in the upper right abdomen that can last hours. Repeated blockages or infection of the gallbladder usually lead to surgical removal, which is one of the most common abdominal surgeries performed.
Diverticular Disease
Diverticular disease develops when small pouches, called diverticula, bulge outward through weak spots in the colon wall. Having these pouches is called diverticulosis, and it’s extremely common after age 40. Most people never know they have them. Problems arise when one or more pouches become inflamed or infected, a condition called diverticulitis.
Diverticulitis causes pain (usually in the lower left abdomen), fever, nausea, and changes in bowel habits. Mild episodes often resolve with rest and a temporary change in diet, while more severe cases may need antibiotics or, rarely, surgery. A diet rich in fiber is generally thought to reduce the risk of developing symptomatic diverticular disease, though the relationship is more complex than once assumed.
Colorectal Cancer and Polyps
Colorectal cancer is the third most common cancer in the U.S., but it’s also one of the most preventable through screening. Most colorectal cancers begin as polyps, small growths on the inner lining of the colon or rectum that are initially benign. Over years, certain types of polyps can accumulate genetic changes and become cancerous. Removing polyps during a colonoscopy breaks that chain.
The U.S. Preventive Services Task Force now recommends that average-risk adults begin colorectal cancer screening at age 45, down from the previous starting age of 50. Screening continues through age 75, with colonoscopy every 10 years being one of several accepted methods. People with a family history of colorectal cancer or a personal history of inflammatory bowel disease may need to start earlier and screen more frequently.
Gastritis and Gastroenteritis
Gastritis is inflammation of the stomach lining. It can be caused by heavy alcohol use, chronic use of certain pain relievers, or infection with a bacterium that thrives in the acidic stomach environment. Symptoms include gnawing or burning stomach pain, nausea, and a feeling of fullness after eating only a small amount. Chronic gastritis can erode the stomach lining over time and increase the risk of stomach ulcers.
Gastroenteritis, often called the “stomach flu,” is an infection of the stomach and intestines usually caused by a virus or contaminated food. It comes on suddenly with vomiting, watery diarrhea, cramps, and sometimes fever. Most cases resolve on their own within a few days, though dehydration is the main concern, especially in young children and older adults.
Symptoms That Need Prompt Attention
Most digestive symptoms are harmless and temporary. But certain red flags point to something more serious. Unexplained weight loss, rectal bleeding, persistent or worsening abdominal pain, a noticeable lump in the abdomen, and anemia (which can show up as unusual fatigue and paleness) all warrant prompt medical evaluation. A change in bowel habits lasting more than six weeks in someone over 60, or a family history of bowel or ovarian cancer combined with new symptoms, also raises the urgency. These don’t necessarily mean cancer or a severe diagnosis, but they indicate something that shouldn’t wait.