The oak tree pathogen, Phytophthora ramorum, has migrated 650 km in 10 years. West Nile virus spread from New York State to 46 other states in 5 years. The difference in the rate of spread is probably related to A) the lethality of each pathogen. B) the mobility of their hosts. C) the fact that viruses are very small. D) innate resistance. E) dormancy viability.