(判断)AIDS, fifty years ago, didn’t exist.Fifteen years ago a few doctors and public health officials noticed the first cases.Within a few years it was clear that it has now killed almost 14 million people around the world. Four years ago doctors came up with the first treatment to make a dent in the spiraling death rate.Today that treatment works for some patients, but it’s not clear how long results will last.And still there is no cure. For the nearly 35 million people around the world now living with HIV, there may never be a cure.Once cells are infected with HIV, it is very difficult — perhaps impossible ―to rid them of the virus.The only sure way to stop AIDS is to prevent infection in the first place, and only a vaccine can do that. Unfortunately HIV is one of the most changeable viruses known to science.After more than a dozen years, it is still rather difficult to produce effective vaccine. Still the billions of dollars spent on AIDS research over the past 20 years has not been wasted.As scientists learn more about how HIV survives in the human body, they are realizing that drugs alone may not be enough.To contain the virus effectively, it may take a balance between drug treatments that can keep HIV levels low and a strengthened immune system that can then target and destroy the remaining virus.Until scientists find a vaccine, however, they may control but never cure the century’s final scourge. 46.AIDS didn’t exist fifteen years ago. 47.Scientists have found a vaccine which can prevent HIV infection. 48.Although some treatment works for some patients with HIV, there is still no cure. 49.HIV is a changeable virus so that it is very difficult to produce effective vaccine. 50.We have wasted billions of dollars on AIDS research.