Helicobacter pylori, a bacterium that that can live in the gut, is the single greatest risk for stomach (gastric) cancer. HOW the bacteria cause the disease is not clear. New research from Swiss and German researchers shows that infection of the gut lining leads to DNA damage in the gut cells. The bacteria don't enter the cells, but just having them attached to the outside seems to be sufficient to cause double-stranded breaks in DNA, shattering the DNA helix into pieces. The researchers showed that most of the damage is repaired but that over longer periods of time, the repair system can be overwhelmed and damage can accumulate. The results sugggest an important way that bacteria can cause changes leading to cancer in their human hosts.