In endemic areas with continuously high PUUV prevalence (e.g., Finland, French Northern Ardennes), strong co-adaptation histories between M. glareolus and PUUV could have selected for higher levels of tolerance to PUUV infections (i.e., lower levels of TNF production) than in populations where prevalence levels of PUUV are low (Czech Republic, French Southern Ardennes). This evidence concerns the gene TNF and infection.