Provocative new research shows that treating people with the AIDS virus can provide a powerful bonus: It cuts the risk that they will infect others.

New infections plummeted in parts of Canada as more people went on AIDS drugs, which lowered the amount of virus they had and the chances they would spread it, the study found.

For every 100 people with HIV who started taking AIDS drugs, new infections dropped by 3 percent in British Columbia, where the study was done. The number of new infections there has been cut in half since 1996, matching a rise in treatment.