This year we have been hearing from local health departments, wildlife rehabilitators, and other observers about an increase in dead bird sightings compared with recent years; somewhat similar to what had been seen earlier in the WNV experience. This might suggest a new WNV strain, either by natural selection or introduction. A new strain could also account for a change in human epidemiology. Alternatively, the reservoir of infection in birds might be substantial—some species experienced considerable mortality when the virus first arrived, but studies indicate that most populations have at least stabilized, if not recovered (5). But we cannot discount the possibility of increased numbers of the Culex species of mosquitoes that are the prime vectors. Culex pipiens, C. restuans, and C. tarsalis are abundant and ubiquitous puddle- and container-breeding mosquitoes broadly distributed geographically in overlapping ranges in urban, suburban, and rural settings. These mosquitoes thrive and reproduce in stagnant, dirty, putrid collections of water in puddles and containers, sewers, storm drains, and catch basins, all of which are part of our residential infrastructure. The drought that has gripped much of the country this year has caused contraction of water sources, creating excellent breeding conditions for these mosquitoes. The record-breaking heat is known to accelerate both the reproductive rate of mosquitoes and the rate of virus replication within them (6). The interplay of heat, drought, human habitats, increased mosquito populations, and enhanced viral development all act in concert to increase the force of transmission. At least, that's the theory. The truth of what lies behind the resurgence of WNV activity this year will take time to uncover.