Japanese microbiologist Satoshi Omura on Monday shared this year's Nobel Prize in medicine for his work on a therapy for debilitating diseases caused by parasitic worms.

The 80-year-old Omura, a professor emeritus of Tokyo's Kitasato University, shared the honor with William C. Campbell of Drew University in New Jersey and Youyou Tu of the China Academy of Traditional Chinese Medicine.

"I'm surprised. I never thought I would win" the prize, Omura reportedly told his daughter Monday evening by phone.