Complex sentence: The most widely known species of the genus is "H. pylori" which is a human pathogen and is responsible for most cases of peptic ulcer, chronic gastritis, duodenitis, and stomach cancer.
Split into simpler sentences: The most widely known species of the genus is "H. pylori". Some strains of this bacterium are pathogenic to humans as it is strongly associated with peptic ulcers, chronic gastritis, duodenitis, and stomach cancer.