BY the middle of the 20th century, Pneumocystis carinii pneumonitis had not been recognized in North America. Yet within the past two decades the prevalence has increased to the point that several hundred cases occur annually in the United States. The emergence of this infectious disease is probably related directly to the increased use of immunosuppressive therapy for diseases of somewhat unrelated causes. Organ-transplant recipients, patients with cancer and children with congenital immune deficiency disorders constitute the majority of patients at risk for the disease. In such cases, referred to here as the "child-adult type," the pneumonitis is an acute, . . .