The 2005 Nobel Prize in Physiology or Medicine was to the guys who discovered that ulcers are caused by bacteria, not stress.
According to the Nobel Prize Committee,
" This year’s Nobel Prize in Physiology or Medicine goes to Barry Marshall and Robin Warren, who with tenacity and a prepared mind challenged prevailing dogmas. By using technologies generally available (fibre endoscopy, silver staining of histological sections and culture techniques for microaerophilic bacteria), they made an irrefutable case that the bacterium Helicobacter pylori is causing disease. By culturing the bacteria they made them amenable to scientific study.
In 1982, when this bacterium was discovered by Marshall and Warren, stress and lifestyle were considered the major causes of peptic ulcer disease. It is now firmly established that Helicobacter pylori causes more than 90% of duodenal ulcers and up to 80% of gastric ulcers. The link between Helicobacter pylori infection and subsequent gastritis and peptic ulcer disease has been established through studies of human volunteers, antibiotic treatment studies and epidemiological studies."
Essentially, before this, everyone with an ulcer kept it for their whole lives. Now, they get treated, and it goes away. They might get another one, but it can be treated too. This was a well deserved win.
Another example of someone who made a difference in a good way?