HIV-infected pneumonia patients with lower CD4 counts exhibit a greater degree of shared bacterial taxonomy in their gut and airway microbiota, and specific gut-enriched taxa belonging to Streptococcus and Veillonella are also detected in the airways of these more severely ill patients, indicating that systemic infection may be perpetrated by a relatively small number of specific genera. The gene discussed is CD4; the disease is susceptibility to pneumonia measurement.