CD4 and acute respiratory distress syndrome: While adaptive immune cell subsets (CD4+ and CD8+ T cells and Foxp3+ regulatory T cells) are not illustrated in Figure 1, their roles in O. tsutsugamushi infection need to be examined, given their important roles in ARDS development in other infection models (Adamzik et al., 2013; Risso et al., 2015; Li et al., 2016).