French researchers Francoise Barre-Sinoussi and Luc Montagnier were cited for their discovery of human immunodeficiency virus, or HIV; while Germany's Harald zur Hausen was honored for finding human papilloma viruses that cause cervical cancer, the second most common cancer among women.
The German medical doctor and scientist received half of the 10 million kronor (US$1.4 million) prize, while the two French researchers shared the other half.
"I'm not prepared for this," zur Hausen, 72, of the German Cancer Research Center in Heidelberg, told The Associated Press by telephone. "We're drinking a little glass of bubbly right now."
In its citation, the Nobel Assembly said Barre-Sinoussi and Montagnier's discovery was one prerequisite for understanding the biology of AIDS and its treatment with antiviral drugs. The pair's work in the early 1980s made it possible to study the virus closely.
That in turn let scientists identify important details in how HIV replicates and how it interacts with the cells it infects, the citation said. It also led to ways to diagnose infected people and to screen blood for HIV, which has limited spread of the epidemic, and helped scientists develop anti-HIV drugs, the citation said.
"The combination of prevention and treatment has substantially decreased spread of the disease and dramatically increased life expectancy among treated patients," the citation said.
The Nobel assembly said zur Hausen "went against current dogma" when he found that some kinds of human papilloma virus, or HPV, caused cervical cancer. He realized that DNA of HPV could be detected in tumors, and uncovered a family of HPV types, only some of which cause cancer.
The discovery led to an understanding of how HPV causes cancer and the development of vaccines against HPV infection, the citation said.
Barre-Sinoussi is director of the Regulation of Retroviral Infections Union at the Institut Pasteur in France, while Montagnier, 76, is the director for the World Foundation for Aids Research in Prevention, also in the French capital.
Barre-Sinoussi's father, Roger Sinoussi, told the AP that his daughter is visiting Cambodia this week.
"I am happy for her," he said, reached at her home in the Paris suburbs.
Alfred Nobel, the Swede who invented dynamite, established the prizes in his will in the categories of medicine, physics, chemistry, literature and peace. The economics prize is technically not a Nobel but a 1968 creation of Sweden's central bank.
The awards include the money, a diploma and an invitation to the prize ceremonies in Stockholm and Oslo on Dec. 10, the anniversary of Nobel's death in 1896.
Nobel left few instructions on how to select winners, but medicine winners are typically awarded for a specific breakthrough rather than a body of research.
Dr. Robert Gallo, director of the Institute for Human Virology at the University of Maryland and a prominent early researcher in HIV, said it was "a disappointment" not to be honored along with Montagnier and Barre-Sinoussi.
But he said all three of the award's recipients deserved the honor. No more than three people can share a Nobel prize.
Maria Masucci, member of the Nobel Assembly, said there was no dispute in the scientific community that the French pair discovered and characterized the virus.
"The dispute is focusing on later events in the history of the virus and in particular on the development of diagnostic tools, which of course is extremely important but basically dependent on the discovery of the virus itself," Masucci said.
Last year's medicine award went to U.S. researchers Mario Capecchi and Oliver Smithies and Briton Martin Evans for work that led to a powerful and widely used technique to manipulate genes in mice, which has helped scientists study heart disease, diabetes, cancer, cystic fibrosis and other diseases.