TLR9 and acute respiratory distress syndrome: Critically ill COVID-19 patients, ALI/ARDS patients and sepsis patients all release high levels of extracellular histones (TLR2, TLR4 and NLRP3 activators [219]); neutrophil elastase (a TLR4 activator [220]) and cell-free DNA (a TLR9 activator [221]), each of which correlate with the probability of the patients being admitted to the intensive care unit and of dying [222].