The Food and Drug Administration approved yesterday the first H.I.V. test that uses saliva rather than blood and delivers results in 20 minutes. Public health officials hope the new test will encourage wider and more frequent testing. About 25 percent of all Americans carrying H.I.V., the virus that causes AIDS, do not know that they are infected, according to estimates by the Centers for Disease Control and Prevention. Around the world, according to the World Health Organization, that figure may be as high as 95 percent. Although the test is as easy to use as a home-pregnancy kit and could...