In the 87 HFRS patients with bacterial infection (42 patients in mild group and 45 patients in severe group), the median serum PCT level in the severe group patients [3.73 ng/ml (range 0.28–62.91 ng/ml)] was higher than that in the mild group patients [0.96 ng/ml (range 0.03–15.44 ng/ml), p < 0.001]. This evidence concerns the gene CALCA and bacterial infectious disease.