Publisher Summary This chapter examines the climatic conditions and natural events that took place in 1992 in southwestern United States and increased the amount of vegetation (especially the piñon nut crop), and hence the population of mice, which ultimately led to an outbreak of a new disease caused by hantaviruses. The numbers of these mice have been boosted by a ten-fold rise in their population size due to intensive agriculture, increasing human exposure to them, and so increasing the risk of Hantaan infection. The chapter also helps us understand the life of rodents that provided the information needed to tie down the cause of this new disease. The virus is very devastating in humans, but apparently doesn't harm the mice. Human beings are of very limited importance in the life of hantaviruses; the virus is difficult to grow from a human source, but from the rodent host the virus can be recovered easily. Because the virus is mainly in the lungs of the mouse, it appears in the saliva, urine, and feces of the mice.