Peptic ulcer disease has been considered to be a “disease of civilisation”. 1 Yet Helicobacter pylori, which now is believed to play a critical role in this illness, 2 3 has probably been part of the human biota since time immemorial, 4 and peptic (especially duodenal) ulceration seems to have become epidemic in humans at a time when H pylori was loosening its firm grip on the human stomach. These phenomena seem contradictory. It is the aim of this paper to describe the elements of the apparent paradox, and to consider several alternative explanations. Resolution of this question has substantial implications for human medicine.