[Reductionism once again. The example of Helicobacter pylori].

The contrasting evidence for a multifactorial pathogenesis of gastroduodenal disease rather than the reductionistic, monocausal role of Helicobacter pylori is presented. Evidence for the former is derived from epidemiological, physiological, immunological and experimental behavioral studies in animals. The high prevalence of the bacterium in popularies and the low incidence of peptic ulcer strongly suggests that it alone cannot play the only pathogenetic role. The evidence that peptic ulcer is not one disease raises the problem of identifying the contributions of psychosocial factors in combination with infection by H. pylori in the various forms of the human disease. The most likely role it plays is that of an "opportunist", when the gastroduodenal mucosal "defense" is compromised by many possible factors.