University of California, Santa Cruz, biologist Marm Kilpatrick, who studies the ecology of infectious diseases, said both the mosquitoes that transmit it and the birds that are important hosts for the virus are abundant in areas that have been modified by human activities.
Therefore, urbanized and agricultural habitats have the highest transmission of West Nile virus, he said.
"The virus has had an important impact on human health in the United States partly because it took advantage of species that do well around people," Kilpatrick said.
The familiar American robin plays a key role in the transmission of West Nile virus across much of North America, he said, so much so he calls robins "super-spreaders" of the virus.
Millions of birds have died from West Nile virus infection, including crows and robins.
"Robins were on a steady upward trajectory thought to be linked to human land use -- they love lawns and agricultural fields," Kilpatrick said. "Crow populations were growing even faster. Now crow populations have crashed downward and robins have leveled off, and we suspect that's due to West Nile virus."