CD4 and AIDS: Other disease conditions in this cohort include chronic obstructive pulmonary disease (COPD; treated with chronic inhaled and systemic steroids), advanced human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS; CD4 cell count of 29 cells/microliter prior to index hospitalization) and critical illness after a motor vehicle accident [requiring placement of an intra-aortic balloon pump (IABP) followed by extracorporeal membrane oxygenation (ECMO)].