FIGURE 2-1 Schematic of infectious disease emergence.


In the complex set of interactions that result in disease emergence, the human element—population growth, density, and distribution; immunosuppression; and behavior—plays a critical role. Increases in the size, density, and distribution of human populations can facilitate the spread of infectious agents; changes in the distribution of populations can bring people into contact with new pathogenic organisms or with vectors that transmit those organisms. Immunosuppression, a by-product of aging, the use of certain medications, diseases, or other factors, often permits infection by microorganisms that are not normally pathogenic in humans. Individual behavior, particularly sexual activity and the use of illegal drugs, contributes to the transmission of a number of diseases that have a major health impact on this and other countries.

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