H. pylori (Helicobacter pylori) is a spiral-shaped bacterium that lives in the stomach lining of roughly half the world’s adult population. Most people who carry it never know they’re infected, but in about 15% of cases, it causes peptic ulcers, and in a smaller fraction, stomach cancer. It’s one of the most common chronic bacterial infections in humans and the primary cause of stomach ulcers worldwide.
How H. Pylori Survives in Your Stomach
Your stomach is one of the harshest environments in the body, with acid strong enough to dissolve food within hours. Most bacteria can’t survive there. H. pylori can, thanks to a clever chemical trick: it produces an enzyme that breaks down urea (a compound naturally present in the stomach) into ammonia and carbon dioxide. The ammonia neutralizes the acid immediately surrounding the bacterium, creating a small protective bubble of near-neutral pH.
This same process does something else remarkable. The stomach lining is coated in thick, gel-like mucus that would normally trap the bacterium in place. But when H. pylori raises the pH around itself, the mucus becomes thinner and less sticky, allowing the bacterium to swim through it using whip-like tails called flagella. It typically has five or six of these, and they’re covered in a protective sheath that may help them resist stomach acid. Once through the mucus, the bacterium burrows into the stomach lining, where it can persist for decades if untreated.
How It Spreads
H. pylori passes from person to person, most often within families and households. The three main routes are fecal-oral (contaminated hands or water), oral-oral (saliva), and gastric-oral (exposure to vomit). CDC research found that exposure to vomit from an infected household member accounted for more than 50% of all new infections in one study, with vomiting carrying a 6.3 times higher transmission risk than diarrhea alone.
Infection rates are strongly linked to living conditions. Crowding, limited sanitation, and poor hygiene increase risk, which is why prevalence is highest in developing regions. In countries that have improved water treatment and household sanitation, infection rates have dropped dramatically over time, following the same pattern as hepatitis A and cholera. Most infections are acquired in childhood, and a global meta-analysis found H. pylori prevalence of about 32% in children compared to nearly 49% in adults.
Symptoms and When They Appear
Most people with H. pylori never develop symptoms. Among infected children, only about 5% to 10% experience any issues at all. When symptoms do occur, the most common one is a dull or burning pain in the stomach, often between meals or during the night when the stomach is empty.
Other possible symptoms include:
- Bloating and excessive burping
- Nausea, sometimes with vomiting
- Loss of appetite or feeling full after eating very little
- Unexplained weight loss
- Indigestion
These symptoms overlap with many other digestive conditions, which is part of why H. pylori often goes undiagnosed for years. The more serious warning signs, like bloody vomit or dark, tarry stools, suggest a bleeding ulcer and need prompt medical attention.
What H. Pylori Does to Your Stomach
The bacterium doesn’t invade deep into the stomach wall. Instead, it sits on the surface of the lining and releases proteins that provoke an immune response. One of these, a toxin called VacA, damages the cells of the stomach lining directly. Another set of proteins triggers chronic inflammation. Over time, this inflammation can erode the protective mucous layer, leaving the underlying tissue exposed to stomach acid.
Where the inflammation concentrates determines what happens next. If it’s mainly in the lower part of the stomach (the antrum), acid production actually increases, which raises the risk of duodenal ulcers, the most common type. If inflammation spreads more broadly across the stomach, acid production drops, which increases the risk of stomach ulcers and, over many years, stomach cancer. About 15% of people carrying H. pylori eventually develop an ulcer, with the outcome depending on the specific bacterial strain, the person’s genetics, and lifestyle factors like smoking.
How It’s Diagnosed
Testing for H. pylori is straightforward and usually doesn’t require any invasive procedures. The two most common noninvasive tests are:
The urea breath test is the most accurate option. You swallow a small amount of specially labeled urea, and if H. pylori is present, the bacterium breaks it down and you exhale the labeled carbon in your breath. This test has a sensitivity of 92% to 94%, meaning it correctly identifies infection in the vast majority of cases.
The stool antigen test detects H. pylori proteins in a stool sample. It’s slightly less sensitive at about 83% but is widely available and easy to repeat after treatment to confirm the infection has cleared.
Endoscopy, where a camera is passed into the stomach, is reserved for situations where a doctor needs to directly examine the stomach lining or take a tissue sample. It’s not a routine first step for most patients.
Treatment and Antibiotic Resistance
H. pylori is curable with antibiotics, but treatment has become more complicated in recent years. The current first-line recommendation from the American College of Gastroenterology is a 14-day course of four medications taken together: an acid-reducing drug taken twice daily, plus three antimicrobial agents taken multiple times per day. This combination is sometimes called bismuth quadruple therapy because one of the four components is bismuth, the same active ingredient in Pepto-Bismol.
The 14-day course is demanding. You’re taking pills multiple times a day, and side effects like nausea, metallic taste, and darkened stools are common. But completing the full course matters, because H. pylori has become increasingly resistant to antibiotics. Globally, resistance to clarithromycin, once a cornerstone of treatment, now exceeds 15% in the majority of countries studied. In parts of Asia, resistance rates for that drug range from 7% to over 90%. Metronidazole resistance is even more widespread. This is a major reason why treatment has shifted toward four-drug regimens and longer courses.
If the first round of treatment fails, alternative regimens exist, but the options narrow with each attempt. Retesting after treatment to confirm the infection is gone is standard practice.
Who Gets Infected and Why It Matters
H. pylori infection is overwhelmingly acquired in childhood, typically before age 10, and persists for life without treatment. A large meta-analysis covering studies from 1970 through 2022 found global adult prevalence around 49%, though rates vary enormously by region. In high-income countries with modern sanitation, prevalence has been falling steadily for decades. In lower-income settings, the majority of the population may still carry the bacterium.
Even though most carriers never develop symptoms, the long-term consequences of untreated infection are significant at a population level. H. pylori is classified as a Group 1 carcinogen, the highest risk category, because of its role in stomach cancer. The progression from infection to cancer is slow, typically unfolding over decades through a sequence of chronic inflammation, thinning of the stomach lining, and precancerous changes. Eradicating the infection interrupts this process, which is why some countries with high stomach cancer rates have implemented population-wide screening and treatment programs.