After pre-entry and entry laboratory studies, patients are followed every 8 weeks until the last patient has completed 104 weeks of study observation. Hepatitis A vaccination is administered at entry and Week 24 to measure responses to neoantigen. [AS PER AMENDMENT 04/26/02: All patients (except those co-enrolled in P1024 on or after November 1, 2001) who have received 2 doses of hepatitis A virus vaccine during the study will be offered an opportunity to enroll in Step II of P1008. Patients in Step II receive a third dose of hepatitis A vaccination at Week 104 or later. Additional blood samples are taken 8 weeks later for antibody detection and peripheral blood mononuclear cell (PBMC) cryopreservation.] All serious bacterial infections that are Grade 3 or higher and OI events are recorded and compared to historical event rates. Virologic and immunologic marker studies are done in all patients and correlated with the risk of developing serious bacterial infections or OI events. Patients are considered to have reached an endpoint if they develop PCP, 2 serious bacterial infections, other Category C OI diagnoses, or CD4% less than 15% and re-initiation of PCP prophylaxis.