West Nile virus (WNV) didn’t wait until the summer to start spreading this year—cases were reported as early as January. As of May 24, the virus had been found in 12 states. It was estimated that WNV would spread across the United States within four years, but many scientists think that it might move faster.

“West Nile virus has been changing and becoming more potent and pathological,” explained Maureen T. Long, DVM, PhD, assistant professor of large animal medicine at the University of Florida’s College of Veterinary Medicine, who treated nearly 50 WNV cases in 2001. “What typically happens (in epidemics) is that the virus is fairly localized and problematic for a couple of years, then it goes away. However, in the U.S., it has marched right down the Eastern seaboard and is going west very rapidly. We have a big mosquito population, and naïve bird and horse populations