PCNA and infection: The density of PCNA-immunoreactive proliferating cells did not differ between runners and control animals 4 days after infection (P = 0.90, Mann–Whitney U-test, n = 19), the median (25th/75th percentile) was 126.3 (55.6/180)/mm2 for runners and 119 (72.3/166.5)/mm2 for controls.