This pattern of spread of Lyme disease and its vectors in the northeastern United States and Europe derives from the recent proliferation of deer, and the abundance of deer derives from the process of reforestation now taking place throughout the North Temperate Zone of the world. Residential development seems to favor small tree-enclosed meadows interspersed with strips of woodland, a" patchiness" much prized by deer, mice, and humans. As a result, increasingly large numbers of people live where risk of Lyme disease and babesiosis is intense. The agents of these infections, that once were transmitted enzootically by an exclusively rodent-feeding vector, have become zoonotic.