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Childhood infections may be necessary to prime the developing immune system in an appropriate manner. In developed countries, the incidence of childhood infections is decreasing, which might explain the observed rise in the prevalence of asthma and other atopic disorders in recent years.To determine whether Helicobacter pylori gastritis, a chronic bacterial infection that is usually acquired in early childhood and then persists throughout life, affects the risk of developing asthma and other atopic disorders.Cross-sectional study of the prevalence of three atopic disorders in 3244 subjects participating in a community-based, prospective, randomized, controlled trial of H. pylori eradication, the Bristol Helicobacter Project. The presence or absence of active H. pylori infection was determined by the13C-urea breath test. The prevalence of asthma, eczema and allergic rhinitis was measured by assessing the use of appropriate medications as surrogate markers for these conditions.There was a 30% reduction in the prevalence of all three atopic disorders in people who had active H. pylori infection, although for each individual atopic disorder the numbers were not quite large enough to reach statistical significance.H. pylori infection is associated with a substantially reduced risk of three common atopic disorders. This is further indirect evidence of the importance of childhood infections in influencing the development of a normal immune response. As such infections become progressively less common in developed countries such as the UK, other methods will need to be developed to try to reduce the risk of atopic disorders.