NOTICE: This ms is the penultimate version, and differs in minor
ways from the published version.--Editor David P. Mindell,1
Jeffrey W. Shultz,2 and Paul W. Ewald3 1Department of Biology
and Museum of Zoology, University of Michigan, Ann Arbor,
Michigan 48109-1079, USA E-mail: email@example.com 2 Department
of Entomology, University of Maryland, College Park, Maryland
20742, USA 3 Department of Biology, Amherst College, Amherst,
Massachusetts 01002, USA Keywords: AIDS, HIV phylogeny,
direction of infection, retrovirus evolution The AIDS pandemic
is a new problem for humans, but it is unclear whether the human
immunodeficiency virus (HIV) giving rise to AIDS is also new to
humans. Either (1) HIV has recently infected humans, in which
case we have a new virus and a new disease, or (2) HIV infected
humans long ago (being mild and/or restricted in range until
recently), in which case we have an old virus and a new disease.
There are precedents for each scenario among known viruses
causing diseases (see Shope and