Ixodes pacificus, known as the western blacklegged tick, is the primary vector of the bacteria that causes Lyme disease in the western United States. A new study by the U.S. Centers for Disease Control and Prevention examined tick abundance and variability in California and found that cold-season temperature and rainfall are the two leading factors that determine an area’s suitability for ticks—deeming the state’s far northern coast and the western Sierra Nevada foothills as the most likely habitat for I. pacificus. (Photo credit: James Gathany, CDC Public Health Image Library)