https://phys.org/news/2018-05-disease-causing-stomach-bug-energy-host.html

"H. pylori infects and causes gastritis in half the world's population. It is transmitted from person to person, usually during the first two years of life," said University of Illinois microbiology professor Steven Blanke, who led the new research. "Long-term infection can extend over decades, and most people never experience any symptoms of infection until the disease has progressed to an advanced state."

The human stomach is the only known environment where H. pylori exists, Blanke said.