Emerging infectious diseases in the 21st century.

The emergence of novel infectious diseases, and the re-emergence of others, is not new. The global ecosystem is constantly changing, influencing the micro- and macroenvironments in which humans and their microbial companions reside and interact. Sometimes the environmental circumstances favour the pathogen and there is an unexpected increase in disease activity or emergence of a new infection. Alternatively, pathogenicity factors are acquired by the microbe, allowing new diseases to emerge or old diseases to increase in importance. The forces that drive the emergence, submergence and re-emergence of infectious diseases are varied, but the influence that humans have on the global ecosystem is often of central importance. This review considers infections that are of particular emerging importance.