H. pylori spreads primarily through saliva and through food or water contaminated with fecal matter. Nearly 44% of adults worldwide carry the infection, and most picked it up during childhood, often from a parent or sibling. The bacterium is remarkably well-adapted to survive in the stomach, which is why once you have it, it rarely goes away on its own.
The Main Routes of Transmission
H. pylori passes between people in two well-established ways. The first is oral-to-oral, meaning the bacteria travel through saliva. Sharing utensils, sharing food, or kissing can transfer it. The second is fecal-to-oral, where the bacteria from an infected person’s stool contaminate water or food that someone else then consumes. In places with untreated water supplies and crowded living conditions, this second route drives much higher infection rates.
The bacterium has been found living in dental plaque, which may act as a reservoir. People who are infected in the stomach are roughly four times more likely to also have H. pylori in their dental plaque compared to people with no stomach infection. This plaque reservoir may also explain why some people get reinfected after treatment.
Why Most Infections Happen in Childhood
Although adults can get infected, the vast majority of cases begin before age five. One study tracking young children found the highest rate of new infections in the two-to-three-year age group, at about 5 per 100 children per year. After age five, new infections dropped sharply, with only one child in the study becoming infected past that age. Around 35% of children and adolescents worldwide already carry the bacteria.
Why children? They put objects in their mouths, they share food more freely, and their immune systems are still developing. But the biggest factor is close contact with infected family members. Having an infected mother or an infected sibling significantly increases a child’s risk. Parents and siblings appear to be the primary source of transmission in most cases, not schools, daycare, or the wider community.
Household Spread and Partners
Living in a crowded household raises your odds of infection by about 38%, based on a large meta-analysis of social factors. This makes sense: more people sharing bathrooms, kitchens, and close quarters means more opportunities for the bacteria to pass between them.
Transmission between spouses is more nuanced than you might expect. In a study of 670 couples, women whose partner was infected had an H. pylori rate of about 35%, compared to roughly 15% among women with an uninfected partner. But when the researchers looked more closely, the picture changed depending on background prevalence. In population groups where infection was already common (above 50%), spousal transmission appeared significant, with an adjusted odds ratio above 6. In groups where infection was uncommon (around 10%), there was no meaningful association between partners’ infection status. In other words, if you live in a setting where H. pylori is rare, catching it from your spouse is unlikely. If you live somewhere the infection is widespread, sharing a household with an infected partner does matter.
How H. Pylori Survives Your Stomach
Your stomach is one of the most hostile environments in the body, with acid strong enough to dissolve metal. H. pylori has a clever workaround. It produces an enzyme called urease, which breaks down urea (a compound naturally present in the stomach) into ammonia. The ammonia neutralizes the acid in a thin layer immediately surrounding the bacterium, creating a pocket of survivable pH.
But surviving the acid is only half the challenge. The stomach lining is coated in a thick, gel-like mucus barrier that would normally trap bacteria in place. H. pylori’s acid-neutralizing trick solves this too. When the pH around the bacterium rises toward neutral, the mucus gel loosens and becomes more liquid. The bacteria, which would otherwise be stuck, can then swim freely through the mucus and attach to the cells of the stomach lining underneath. Once attached, the infection can persist for decades if untreated.
Water, Food, and Environmental Factors
Contaminated water is one of the clearest environmental risk factors, particularly in developing countries. Lacking a reliable supply of treated, clean water increases exposure to fecal contamination carrying the bacteria. The combination of untreated water, crowded housing, and limited sanitation infrastructure explains why H. pylori prevalence in some regions of Africa and South America exceeds 70%, while in parts of Western Europe it sits closer to 20%.
Food can also carry the bacteria when handled by someone who is infected and hasn’t washed their hands properly, or when produce is washed or irrigated with contaminated water. There is no strong evidence that any particular type of food is more likely to harbor H. pylori. The risk comes from the hygiene conditions during preparation, not from the food itself.
Why Geography Matters More Than You’d Think
When researchers pooled data from dozens of studies on social factors and H. pylori, they found that geographic region was the single most consistent factor explaining differences in infection rates. Overcrowding, water quality, and hygiene all played roles, but their importance varied dramatically depending on where in the world the study took place. This suggests that H. pylori transmission is shaped by overlapping local conditions (infrastructure, cultural practices, climate, population density) rather than any one universal risk factor. Two people with identical household sizes and water sources can face very different risks depending on the country they live in.