Emerging and re-emerging infections

The microbial world is complex, dynamic and constantly evolving. Infectious agents reproduce rapidly, mutate frequently, cross the species barrier between animal hosts and humans, and adapt with relative ease to their new environments. Because of these traits, infectious agents are able to alter their epidemiology, their virulence, and their susceptibility to anti-infective drugs. When disease is caused by a microbe that is newly identifi ed and not known previously to infect humans, it is commonly called an emerging infectious disease, or simply an emerging infection. When disease is caused by an infectious agent previously known to infect humans that has re-entered human populations or changed in epidemiology or susceptibility to anti-infective drugs, it is called a re-emerging infection. A report published by the United States Insti tute of Medicine in 1992 fi rst called attention to emerging and re-emerging infectious diseases as evidence that the fi ght against infectious diseases was far from won, despite great advances in the development of antimicrobials and vaccines (Lederberg et al. 1992). All forms of infectious agents—bacteria, viruses, parasites, and prions—are able to emerge or re-emerge in human populations, and it is estimated that 70 per cent or more of all emerging infections have a source in animals. When a new infectious agent enters human populations there are several potential outcomes. In some instances, infected humans become ill, while in others, infections are asymptomatic. Once humans are infected, human-to-human transmission may or may not occur. If it occurs, it may be limited to one, two, or more generations, or it may be sustained indefi nitely. Among those infectious agents that cause disease, some maintain their virulence, while others attenuate over time. Changes in the epidemiological characteristics of infectious agents may occur gradually, or they may occur abruptly as the result of a sudden genetic change during reproduction and/or replication.