ERVW-1 and infection: Three major questions were addressed: (i) Compared to the SIVmac251 virus inoculum, are few SIV envelope variants transmitted orally?, (ii) Is the lower viremia and better clinical outcome of vaccinated infants related to the initial genetic diversity of SIV env quasispecies?, and, (iii) Is the evolution of SIV envelope quasispecies during the course of infection associated with the development of SIV neutralizing antibody?