Microorganisms are everywhere and are essential to the existence of life on our planet, although only a few are known to cause disease in humans. Selected viruses and bacteria have become useful models for laboratory investigations.

Infectious diseases continue to constitute a serious global threat to human health and well-being, representing the leading cause of death worldwide and the third leading cause in the United States. Due to dramatic changes in society, technology, and the environment, plus the reduced effectiveness of previous approaches to disease control, the spectrum of infectious diseases is expanding.

Diseases once believed to be conquered are increasing, as pathogens evolve and spread and new agents are recognized. Surprisingly, some diseases not previously recognized as being infectious have been found to have a microbial etiology. The term "emerging infectious diseases" refers to these phenomena.

Identifying important microbial agents and their mechanisms of pathogenesis has highlighted the continuing importance of infectious diseases to the public health and the need to reassess old concepts of microbial disease causation. In addition, we must assume that additional agents will be identified using powerful new technologies.