H. pylori infections are caused by a spiral-shaped bacterium called Helicobacter pylori that spreads from person to person, primarily through contact with saliva, vomit, or fecal matter. About 44% of adults worldwide carry the infection, and most people pick it up before age 10. The bacterium is remarkably well-adapted to life in the stomach, where almost nothing else can survive, and once it takes hold it typically stays for life unless treated.
How H. Pylori Spreads
The exact transmission route is still debated, but person-to-person spread within families is the most common pattern. There are three main pathways: mouth-to-mouth contact, exposure to vomit or stomach contents, and the fecal-oral route. No single pathway has been identified as dominant, and different routes likely matter more in different settings.
Mouth-to-mouth transmission is especially relevant between mothers and young children. The bacterium colonizes the mouth after reflux or vomiting, and it has been cultured directly from saliva, dental plaque, and the tissue beneath the gumline. A mother chewing food before giving it to an infant, or sharing utensils, can pass the bacteria along. Among children, sharing cups or close contact at school creates similar opportunities.
The fecal-oral route matters most where sanitation is poor. H. pylori DNA shows up frequently in human stool, and the bacterium has been isolated from untreated municipal wastewater. In one study of children in Lima, Peru, those from high-income families whose homes used municipal water were 12 times more likely to be infected than those whose water came from community wells, suggesting contaminated water infrastructure can be a major source regardless of household wealth.
Contaminated Water and Food
H. pylori has been detected in drinking water, seawater, vegetables, milk, and meat. The bacterium can survive in refrigerated milk for 5 to 12 days, in ground beef for about a week at fridge temperature, and in vegetables like lettuce and carrots for 3 to 5 days. In a large survey of ready-to-eat foods, H. pylori was found in 74% of samples, with olive salad, restaurant salad, fruit salad, and soup among the most commonly contaminated items.
Raw and undercooked animal products also carry risk. Studies have isolated H. pylori in 25% of cow meat samples, 37% of sheep meat, and detected its DNA in 44% of ready-to-eat raw tuna. These findings support the idea that food handled under poor hygienic conditions is a meaningful transmission vehicle, not just a theoretical one.
Why Children Are Most Vulnerable
Most new H. pylori infections happen before age 10, with a median age of acquisition around 7.5 years. Children are more susceptible for practical reasons: they have more hand-to-mouth contact, less established hygiene habits, and closer physical contact with caregivers who may carry the bacteria. In developing countries, childhood infection rates remain high, with about 35% of children and adolescents testing positive globally.
Once acquired in childhood, the infection almost always persists into adulthood. The immune system mounts an inflammatory response but cannot clear the bacteria on its own, which is why H. pylori requires antibiotic treatment to eliminate.
How the Bacterium Survives Stomach Acid
The stomach is one of the most hostile environments in the human body, with a pH low enough to kill most microorganisms on contact. H. pylori gets around this by producing an enzyme that breaks down urea, a compound naturally present in the stomach, into ammonia and carbon dioxide. The ammonia neutralizes the acid in the bacterium’s immediate surroundings, creating a protective chemical buffer. This buffer maintains the space around the bacterium at a pH of about 6.1, close to neutral, even while the rest of the stomach remains highly acidic.
This acid-neutralizing trick is not optional for the bacterium. It is the core survival mechanism that allows H. pylori to colonize a niche no other common pathogen can occupy. Every known strain of H. pylori carries the genes for this process, and strains that lose this ability cannot establish infection.
What Makes Some Strains More Dangerous
Not all H. pylori infections cause the same degree of harm. The bacterium produces two key proteins that determine how much damage it does to the stomach lining. One is an oncoprotein (directly demonstrated to cause gastrointestinal tumors in animal models) that gets injected into stomach cells and disrupts their normal function. The other is a toxin that punches pores in cells and sabotages their internal cleanup systems.
These two proteins work together in a way that amplifies the damage. Normally, your stomach cells would break down the cancer-promoting protein through their internal recycling process. But the pore-forming toxin disables that recycling machinery, allowing the dangerous protein to accumulate inside cells instead of being destroyed. This buildup over years and decades is one mechanism behind H. pylori’s link to stomach cancer. Every H. pylori strain carries the gene for the pore-forming toxin, suggesting it plays a central role in the bacterium’s ability to persist long-term.
Risk Factors That Increase Your Odds
The strongest predictors of H. pylori infection are the conditions you grew up in, not your current lifestyle. Household crowding during childhood, limited access to clean water, and lower sanitation standards all increase the likelihood of early exposure. These factors explain why infection rates vary dramatically by region: prevalence remains above 50% in many parts of Africa, South America, and Asia while falling below 30% in parts of Western Europe and North America.
Genetics also play a role. Studies of identical twins show higher matching infection rates than fraternal twins, even after accounting for shared living conditions. A large meta-analysis identified 47 genetic variants across 18 locations in the human genome that influence susceptibility to H. pylori. Several of these variants affect immune system genes involved in recognizing bacteria, which may explain why some people exposed to the same household conditions get infected while others do not.
What Happens if the Infection Persists
The majority of people with H. pylori never develop symptoms. The bacterium causes chronic low-grade inflammation in the stomach lining of virtually everyone it infects, but only a fraction progress to clinical disease. About 10% of infected people develop peptic ulcers. Between 1% and 3% develop stomach cancer. Less than 0.1% develop a rare type of stomach lymphoma.
These percentages sound small, but given that roughly 3.5 billion people carry the infection worldwide, they translate into enormous numbers of ulcer and cancer cases. H. pylori is classified as a definite carcinogen, and in long-term studies, stomach cancer developed in about 3% of infected patients compared to none of the uninfected patients followed over the same period.
Reducing the Risk of Transmission
Because H. pylori spreads through contaminated hands, mouths, water, and food, the most effective prevention measures are basic hygiene practices. Regular handwashing, particularly before meals and after using the bathroom, is considered one of the most effective ways to reduce transmission. Maintaining good oral hygiene matters because the mouth serves as a secondary reservoir for the bacterium.
Within families, using serving utensils rather than shared chopsticks or spoons, serving meals on individual plates, disinfecting toilets regularly, and drinking boiled or properly treated water all reduce spread. These measures are especially important in households with young children, since childhood is the window when most infections are acquired. In areas where municipal water quality is uncertain, boiling drinking water is a straightforward way to eliminate the bacterium before it reaches the stomach.