The oak tree fungal pathogen, phytophthora ramorum, has migrated eight hundred kilometers in fifteen years. west nile virus spread from new york state to forty-six other states in five years. the difference in the rate of spread is probably related to

I believe the difference in the rate of spread is probably related to the mobility of their hosts. Natural populations of hosts and parasites are often subdivided and patchily distributed such that some regions of a host species' range will be free from a given parasite. The mobility of the hosts greatly influences the distribution of a given parasite.