People who started anti-AIDS drugs while their immune system was strong were far less likely to develop AIDS or other serious illnesses than if they waited until blood tests showed their immune system was starting to weaken, the U.S. National Institutes of Health announced Wednesday. HIV may not trigger symptoms for years, raising the question of how soon after diagnosis patients should begin taking expensive medications that may cause side effects. The START trial — Strategic Timing of AntiRetroviral Treatment — sought proof by randomly assigning still healthy patients either to receive early therapy or to delay therapy until their CD4 cells, a key sign of immune system health, dropped into a worry zone. Researchers tracked deaths, the development of AIDS-related illnesses and the development of serious non-AIDS events such as cancer, heart disease and kidney or liver disease.