S100A9 and viral pneumonia: Moreover, S100A9 protein levels were positively correlated with C reactive protein (CRP) in BALF of patients with bacterial but not viral pneumonia (Pearson’s r = 0.54, p-value = 0.03 for bacterial versus viral pneumonia with Pearson’s r = -0.76, p-value = 0.01) (S1A and S1B Fig).