In 2005 , the Karolinska Institute in Stockholm gave the Nobel Prize in Physiology or Medicine to Dr. Marshall and Dr. Warren for finding the bacterium Helicobacter pylori and its part in gastritis and peptic ulcer disease . In 2005 , the Karolinska Institute in Stockholm awarded the Nobel Prize in Physiology or Medicine to Marshall and Robin Warren , his long-time collaborator , `` for their discovery of the bacterium Helicobacter pylori and its role in gastritis and peptic ulcer disease '' .