Viral LoadHIV InfectionsHIV-1Antiretroviral Therapy, Highly ActiveCD4 Lymphocyte CountRNA, ViralAnti-HIV AgentsDNA, ViralReproducibility of ResultsViremiaTime FactorsAnti-Retroviral AgentsHIVSimian Acquired Immunodeficiency SyndromePlasmaSimian immunodeficiency virusAntiviral AgentsProspective StudiesCohort Studies