Final answer:
H. pylori bacteria is the primary cause of peptic ulcers, as it is able to tolerate the acidic environment of the stomach. Transmission can occur through contaminated food, water, or saliva. It is found in the majority of stomach and duodenal ulcers.
Step-by-step explanation:
The primary cause of peptic ulcers is H. pylori bacteria. This gram-negative bacterium is able to tolerate the acidic environment of the human stomach and has been shown to be a major cause of peptic ulcers. H. pylori infection can be transmitted through contaminated food, water, or saliva. It is found in approximately 80% of stomach ulcers and over 90% of duodenal ulcers.