Infectious pathogens have undoubtedly been a significant factor throughout the evolutionary history of humans, causing illness and death. It is equally certain that exposure to infectious agents and illness from infection became a more serious problem for human groups during the past 10,000 years. During this period animals and plants were domesticated, giving rise to an agricultural economy that provided the resources that stimulated sedentism and urbanism to become important environmental realities in human life. The risk of infectious disease was enhanced by exposure to pathogens carried by domestic animals (zoonoses) as well as increased human-to-human transmission of pathogens in larger concentrations of people.