Dancing MatrixBy: Robin Marantz Henig Even as humanity reels beneath the assault of AIDS, epidemiologists are gearing themselves up for the plague's successor. It might be dengue fever, whose carrier, the Asian tiger mosquito, has recently appeared in the United States, or Creutzfeldt Jakob disease, which has been transmitted by contaminated human growth hormone. The next pandemic might be caused by any of a dozen viruses that were once confined to other species or