Since it jumped the Atlantic and landed in New York in 1999, the West Nile virus has spread to all of the lower 48 United States. The West has been especially hard hit by the mosquito-borne disease. In 2007, for example, the 11 Western states reported nearly 1,600 cases of the West Nile Virus, more than half of all cases in the country. About 20 percent of the Western cases were of the more serious or “neuroinvasive” variety. At least 28 people died in the West from West Nile infection during 2007.

A new strain of West Nile – first seen in 2002 and particularly well suited to hotter weather – has overtaken the original bug, according to researchers at the Centers for Disease Control. The new strain replicates faster in mosquitoes when it’s warm. Despite big snow-pack and huge runoff across the West, cooler temperatures and less rain early in the season have made 2008 a year of declining cases of the West Nile virus. More than halfway through the peak season for infection, only half the Western states have reported human cases.