The study includes 250 children, 3 months to 12 years of age. All participants receive oral AZT. IVIG or intravenous placebo is administered every 28 days. Patients are followed for the development of serious bacterial infection, as well as for a number of factors relating to safety, tolerance, progression of disease, and survival. This is an outpatient study conducted over a minimum 100-week period. The children are evaluated every 2 weeks for the first 8 weeks, and monthly thereafter.