Despite the fact that H. pylori infection is one of the most common infectious diseases on earth, the way in which it is transmitted is not precisely known. There are several reasons for this: First, there are numerous sources of infection. Second, people tend not to see their doctors when initially infected, so physicians are unable to precisely pinpoint where and how their patients became exposed. Finally, the symptoms, which are usually mild, are common to any number of other infections (such as the stomach flu). In fact, H. pylori infection is often diagnosed only when other diseases have been eliminated, a process that can take years.

Scientists have gotten around this mystery, at least somewhat, by studying the rate of infection in populations, hoping to find clues about what factors cause the infection to spread. These studies, called epidemiologic studies, are one way to study diseases before much is known about how they occur. Epidemiologists, who are scientists trained in the mathematical analysis of health data, work to determine causes of disease by examining data from large populations of people. They gather huge amounts of information from a wide array of sources and then perform mathematical analyses to determine whether there is any statistical, or mathematical, association between a given factor and the presence of the disease.

Epidemiologists make use of government health statistics, medical and death records, and personal interviews with patients and physicians to piece together the complex clues that tell the story of the disease-causing organism and its victims. They gather data about everything they can think of, such as the person’s age, educational level, occupation, neighborhood, family size, h pylori diet, number and type of pets, marital status, and family medical history, to name a few. Then, by putting these clues together, the epidemiologist tries to identify things that infected people have in common, and how an infected person’s lifestyle brings him or her into contact with the organism. Individually, factors like age, gender, or income seem very general and do not reveal much about the disease, but when placed together a pattern begins to emerge.

The pattern of infection in developed countries differs markedly from that of developing countries, and the underlying causes of infection in developed countries are less clearly defined. In the US, Canada, and Western Europe the rate of infection is comparably quite low. The frequency of infection in these countries increases with age, suggesting that the most common mode of transmission is probably not parent to child. The practice of water chlorination and the widespread availability of flush toilets and sewage disposal systems reduce the opportunity for infection in the general environment.

While the overall rate of infection is lower in developed countries, it is not uniformly lower. There are pockets of infection in developed countries, some of which are associated with poverty, the strongest factor determining the pattern of infection. Also, immigrant populations whose origins are in developing countries typically have higher rates of infection than the general population. For example, in a recent study of H pylori infection rates of Mexican-Americans, the rate of infection among recent Mexican immigrants was similar to the rate of infection in Mexico, but among Mexican-Americans born in the U.S., the rate of infection was more similar to the U.S. population- at-large.