MKI67 and infection: The frequencies of PD-1+ and Ki-67+ cells in the lung tissue were significantly affected by infection but not by age, i.e., were significantly higher in the infected young (6.2% and 1.4%, respectively) and old groups (6.1% and 12.9%, respectively) compared with the healthy young (3.1% and 6.7%, respectively; p < 0.05) and old groups (3.4% and 7.4%, respectively; p < 0.05) (Fig. 4c).