H. pylori (Helicobacter pylori) is a type of bacteria that lives in the lining of the stomach. It infects roughly 44% of adults worldwide, making it one of the most common bacterial infections on the planet. Most people who carry it never know they have it, but in some cases it causes stomach ulcers, chronic inflammation, and raises the risk of stomach cancer.
How H. Pylori Survives in Your Stomach
Your stomach is an intensely acidic environment, hostile to nearly all bacteria. H. pylori gets around this with a clever chemical trick. It produces an enzyme that breaks down urea (a natural compound in the stomach) into ammonia and carbon dioxide. The ammonia neutralizes the acid immediately surrounding the bacterium, creating a small protective buffer zone. This enzyme becomes 10 to 20 times more active as acidity increases, meaning the harsher the environment gets, the better H. pylori defends itself.
The bacterium is also shaped like a corkscrew and has whip-like tails that let it burrow into the thick mucus lining of the stomach wall. Once embedded there, it’s shielded from both stomach acid and your immune system. It can persist for decades if untreated.
How It Spreads
H. pylori passes between people, most often within households and during childhood. The main routes are fecal-oral (poor hand hygiene after using the bathroom), oral-oral (shared saliva), and through vomit. CDC research found that exposure to vomit from an infected household member accounted for over 50% of all new infections in one study, with vomiting carrying a 6-fold increased risk of transmission compared to other exposures.
Crowded living conditions and limited access to clean water increase the risk significantly. This is why infection rates are higher in lower-income regions and why most people acquire the bacteria during childhood, when hygiene practices are less consistent and close contact with family members is constant.
Symptoms of H. Pylori Infection
Most H. pylori infections produce no symptoms at all. People can carry the bacteria for years without realizing it. When symptoms do appear, they typically stem from inflammation of the stomach lining (gastritis) or a peptic ulcer that the bacteria helped create. Common signs include:
- A burning or aching pain in the upper abdomen, often worse on an empty stomach
- Bloating and frequent burping
- Nausea or loss of appetite
- Unexplained weight loss
The hallmark complaint is stomach pain that flares when you haven’t eaten and eases after a meal or an antacid. If you notice dark or tarry stools, vomiting that looks like coffee grounds, or severe persistent pain, those suggest a bleeding ulcer and need prompt medical attention.
Long-Term Health Risks
Left untreated for years, chronic H. pylori infection can lead to more serious problems. Peptic ulcers are the most common complication, forming in the stomach or the upper part of the small intestine. The bacterium damages the protective mucus layer, leaving the tissue underneath exposed to digestive acid.
The bigger concern is cancer. The majority of stomach cancers (gastric adenocarcinoma) and a rare type of stomach lymphoma called MALT lymphoma are attributed to H. pylori infection, according to the National Cancer Institute. Nearly all patients diagnosed with gastric MALT lymphoma show signs of H. pylori, and treating the infection alone can sometimes put the lymphoma into remission. The absolute risk of any individual developing stomach cancer remains low, but chronic infection is the single largest risk factor for it.
How H. Pylori Is Diagnosed
The most common non-invasive test is the urea breath test. You swallow a small capsule or drink containing a special form of urea, and if H. pylori is present, the enzyme it produces breaks down that urea and releases carbon dioxide that can be detected in your breath about 15 to 30 minutes later. The test is highly accurate, with sensitivity and specificity rates above 95% in most settings.
A stool antigen test is another reliable option that detects H. pylori proteins in a stool sample. It’s widely available and often used when the breath test isn’t practical. Blood antibody tests exist but are less useful because they can stay positive long after the infection has been cleared, so they can’t distinguish between a current and past infection. If you’re having an endoscopy for other reasons, tissue samples from the stomach lining can also be tested directly.
For any of these tests to be accurate, you generally need to stop taking acid-reducing medications (like proton pump inhibitors) for at least two weeks and antibiotics for at least four weeks beforehand, since these can suppress the bacteria enough to produce a false negative.
Treatment and Antibiotic Resistance
H. pylori is treated with a combination of multiple antibiotics taken alongside an acid-suppressing medication, typically for 14 days. The current recommended first-line approach uses four drugs together: an acid reducer, two different antibiotics, and a bismuth compound (the active ingredient in Pepto-Bismol). This “quadruple therapy” is necessary because H. pylori has become increasingly resistant to individual antibiotics.
Antibiotic resistance is a genuine and growing problem with this bacterium. Resistance to clarithromycin, once a mainstay of treatment, now exceeds 15% in the majority of countries studied. In parts of Asia, clarithromycin resistance ranges from 7% to over 92%. Metronidazole resistance is even more widespread globally. Because of this, the older and simpler three-drug regimen built around clarithromycin is no longer recommended unless lab testing confirms the bacteria are sensitive to it.
Treatment typically eradicates the infection in 80% to 90% of cases on the first attempt. If the first round fails, your doctor will usually switch to a different antibiotic combination for the second try. A follow-up breath test or stool test is generally done at least four weeks after finishing treatment to confirm the bacteria are gone. Reinfection after successful treatment is uncommon in developed countries, occurring in roughly 1% to 2% of people per year.