The bacteria known as H. pylori are depicted by the CDC as a terrifying and fast-spreading germ. A 1998 fact sheet says that infection causes ulcers and stomach cancer, and that it afflicts two-thirds of humans worldwide.
http://www.bloombergview.com/articles/2016-01-07/turns-out-h-pylori-is-an-ancient-bug-and-also-a-feature
This Ancient Stomach Bug Was Also a Feature