Most people pick up H. pylori during early childhood, typically before age 5, through close contact with an infected family member. The bacteria spread through vomit, stool, and saliva, and roughly half of all adults worldwide carry the infection. The good news: 80 to 90 percent of infected people never develop symptoms.
Person-to-Person Contact Is the Main Route
H. pylori passes between people through three well-established pathways: fecal-oral (touching contaminated surfaces or objects and then touching your mouth), oral-oral (through saliva), and gastric-oral (through vomit). Of these, exposure to vomit from an infected person appears to be the single biggest driver. A CDC-published study tracking transmission within households found that contact with an infected person’s vomit explained more than 50 percent of all new infections and over 70 percent of the most clearly documented cases.
This means something as ordinary as a child vomiting from a stomach bug can spread H. pylori to other family members, especially in close quarters. The bacteria have been reliably recovered from both vomit and stool during bouts of gastroenteritis, making any episode of stomach illness in an infected person a prime opportunity for spread.
Childhood Is the High-Risk Window
A longitudinal study tracking children over time found that the highest rate of new infection occurred in the 2-to-3-year age group, at about 5 new infections per 100 children per year. After age 5, the risk dropped sharply. Only one child in the entire study became infected after that age. Three factors independently raised a young child’s risk: having an infected mother, having an infected older sibling, and still using a bottle past age 2.
This pattern makes sense when you consider how toddlers live. They share utensils, put objects in their mouths, need help with hygiene, and are in constant close physical contact with caregivers. In developing countries, where sanitation infrastructure is limited and households tend to be larger, childhood infection rates are especially high.
Contaminated Water and Food
H. pylori can also behave like a waterborne or foodborne pathogen. The bacteria survive for several days in water, vegetables, and animal-derived foods. A systematic review of food and water samples in Iran found H. pylori in about 11 percent of items tested, including produce and animal products. In regions where tap water is untreated or where produce is washed with contaminated water, this becomes a meaningful route of exposure.
For most people in countries with modern water treatment, this route is less of a concern. But if you’re traveling to areas with unreliable water infrastructure, contaminated food and water become a realistic way to encounter the bacteria.
Pets May Play a Role
Dogs and cats can harbor H. pylori and related species in their stomachs, often with mild or no symptoms. One study using stool testing detected H. pylori antigens in 93 percent of dog fecal samples and about 27 percent of cat fecal samples from households where the infection was circulating. Researchers have found similar bacterial strains in pets and their owners, suggesting that companion animals can act as reservoirs. The risk is likely highest for people who have very close contact with their pets, such as sharing sleeping spaces or allowing face licking, though the exact contribution of pets relative to human-to-human spread is still being worked out.
Dental Plaque and Reinfection
H. pylori doesn’t just live in the stomach. It has been detected in dental plaque and saliva, with oral prevalence rates ranging from about 5 to 83 percent across different study populations. This matters for two reasons. First, it supports the oral-oral transmission route: kissing, sharing utensils, or pre-chewing food for a child can transfer the bacteria. Second, the mouth may act as a hiding spot that reinfects the stomach even after successful antibiotic treatment. Some researchers believe eradicating H. pylori from the oral cavity is actually harder than clearing it from the gut, which could explain why some people test positive again after treatment.
How the Bacteria Survive Stomach Acid
One reason H. pylori is so persistent once you have it is its remarkable acid-survival trick. Your stomach maintains a pH around 1 to 2, acidic enough to destroy most bacteria on contact. H. pylori produces an enzyme that rapidly breaks down urea (a natural compound present in the stomach) into ammonia. That ammonia neutralizes the acid immediately surrounding the bacterium, creating a tiny protective bubble. When acid levels rise, the bacteria ramp up this process even further. This lets H. pylori survive for hours in conditions that would kill nearly any other organism, giving it time to burrow into the protective mucus lining of the stomach wall where it sets up a long-term home.
Risk Factors That Increase Your Chances
Several conditions make infection more likely:
- Crowded living conditions. More people in a household means more opportunities for the bacteria to pass between family members, particularly young children.
- Living with an infected person. Having an H. pylori-positive mother or older sibling is one of the strongest predictors of childhood infection.
- Limited sanitation. Inadequate sewage systems and untreated drinking water create environmental reservoirs.
- Poor hand hygiene. The fecal-oral route depends on the bacteria reaching your mouth from contaminated hands or surfaces.
These factors overlap heavily with socioeconomic status, which is why infection rates are significantly higher in developing countries. Global data shows about 49 percent of adults carry the infection, though that number has been declining slowly over the past three decades as sanitation and living standards improve in many regions.
Most Infections Cause No Symptoms
If you’re worried about having picked up H. pylori, it helps to know that the vast majority of infected people never get sick from it. Between 80 and 90 percent of carriers remain completely asymptomatic throughout their lives. The remaining 10 to 20 percent develop problems ranging from chronic gastritis and ulcers to, in rare cases, stomach cancer. Why some people get sick and others don’t isn’t fully understood, but it likely involves a combination of the specific bacterial strain, the person’s immune response, and other individual factors like diet and genetics.
Reducing Your Risk
Since H. pylori spreads through bodily fluids and contaminated food or water, prevention comes down to basic hygiene. Wash your hands thoroughly before eating and after using the bathroom. When traveling in regions with limited water treatment, stick to bottled or boiled water and avoid raw produce washed in tap water. If someone in your household has chronic stomach symptoms, getting them tested and treated reduces the chance of spreading the bacteria to other family members. Treatment typically involves a course of antibiotics combined with an acid-reducing medication, and completing the full course is important because partial treatment can leave behind resistant bacteria.