Nowak et al. demonstrates that decreased α-diversity measured as the number of bacterial taxa in fecal samples is observed in the highly immune-deficient group (CD4+ T cell counts between 120 and 150 cells/μl) compared to other individuals with chronic untreated infection17, indicating a potential role for a breakdown in colonization resistance in opportunistic infections that occur with advanced disease. This evidence concerns the gene CD4 and Opportunistic infection.