TLR9 and COVID-19: Herein, we found that the excessive ncfDNA and mtcfDNA in COVID-19 plasma caused the overproduction of mtROS in kidney tubule cells via TLR9, which might contribute to a positive feedback loop that contributes to the pathogenesis of COVID-19 multiple organ failure; that is, tissue injury leads to the release of cfDNA, which then acts on other tissue types via TLR9 to trigger additional tissue injury.