Figure 1.

Major microbial factors in acute respiratory episodes. To the left of the figure, the circulation of multiple respiratory viruses in the population provides a continued source of mucosal insults. Circulation and adherence of common bacterial pathogens has the potential to cause invasive disease and sepsis as well as lower-grade chronic damage. Airway commensals also circulate within the population, and their diversity is protective against infection. Viruses and bacteria interact positively and negatively within the mucous layer and the epithelium. The microbiota and epithelia induce reactive immunity to infection and consequent inflammation, shown on the right. Current therapies and research investment are directed to the right, but consequences of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic show the extraordinary potential of left-sided interventions.