A study has found that treating AIDS patients with antiretroviral drugs doesn’t just help patients: infected people under treatment are far less likely to transmit HIV—a result that could have major implications for slowing the spread of the disease. The trial covered nearly 1,800 couples in nine countries in which one partner had HIV. Patients taking the antiretroviral drugs used to treat the virus were 96.3 percent less likely to pass on the virus. And patients who began taking the drugs sooner were much less likely to pass it on than those who waited. The findings were so clear that a panel monitoring them decided they should be released four years ahead of schedule.