The great diversity of infectious pathogens, their possible hosts, and the ways in which their hosts respond to infection has resulted in multiple definitions for "natural reservoir", many of which are conflicting or incomplete. In a 2002 conceptual exploration published in the CDC's "Emerging Infectious Diseases", the natural reservoir of a given pathogen is defined as "one or more epidemiologically connected populations or environments in which the pathogen can be permanently maintained and from which infection is transmitted to the defined target population." The "target population" is the population or species in which the pathogen causes disease; it is the population of interest because it suffers from disease when infected by the pathogen (for example, humans are the target population in most medical epidemiological studies).