[Note that follow up of patients enrolled in Phase 1 will continue during this time period, but is considered to be part of Phase 1 rather than Phase 2. Also, sites included in Phase 2 may change over time as a result of the instability of CD4 testing (e.g., if participating laboratories stop conducting CD4 testing, those sites might no longer be included; if participating laboratories begin CD4 testing for other sites, those sites might be included).]