Bacteria that benefit human beings when they are present in the body are called commensals  The bacterium Helicobacter pylori plays a primary role in the development of stomach ulcers  But since stomach ulcers occur in less than 10 percent of those harboring H  pylori, and since it allegedly strengthens immune response, many scientists now consider it a commensal  But this is surely misguided  Only about 10 percent of the people who harbor Mycobacter tuberculosis -- a bacterium that can cause tuberculosis -- get sick from it, yet no one would call M
tuberculosis a commensal
Stomach ulcers caused by H. pylori and tuberculosis can both be effectively treated with antibiotics.