U.N. experts compiling figures on HIV infection worldwide were stunned to learn that, over the past two years, they had missed millions of Africans with AIDS, an official with the U.N. AIDS program in Geneva said. A year ago, the agency recalculated the figures it released for 1996 and realized that the figures for Africa were incorrect. The real number of people living with AIDS in sub-Saharan Africa was closer to 18.6 million--not 14 million, as previously estimated. The agency concedes that for many countries, it can still only make estimates. Determining the number of people who have died from AIDS is also a problem.