virus (the enterically transmitted non-A, non-B hepatitis that is usually waterborne and is widespread in tropical areas including parts of South America) has tentatively been classified as a calicivirus (Reyes et al., 1990).
There are also a number of established diseases whose link to an infectious agent has only recently been discovered. In addition to peptic ulcer, mentioned in Chapter 1, other diseases with a newfound link to a microbe include cervical cancer (associated with human papillomaviruses) and human T cell lymphotropic virus (HTLV)-I-associated myelopathy or tropical spastic paraparesis (resulting from infection with HTLV). Diseases for which possible links to infectious agents are under investigation include rheumatoid arthritis (parvovirus B19, HTLV-I); atherosclerosis (cytomegalovirus [CMV], herpes simplex virus [HSV]-1 and HSV-2, or Chlamydia pneumoniae); and insulin-dependent diabetes mellitus (coxsackievirus B5). Several of these examples are discussed later in this chapter.
Rather than categorize emerging microbial threats by type of agent—viral, bacterial, protozoal, helminthic, or fungal—this report classifies emerging threats according to the factors related to their emergence:
Human demographics and behavior
Technology and industry
Economic development and land use
International travel and commerce
Microbial adaptation and change
Breakdown of public health measures
The classification draws attention to the specific forces that shape infectious disease emergence (see Figure 2-1). These forces (i.e., factors in emergence) operate on different elements in the process of emergence. Some of the factors influence the acquisition of an emerging microbe by humans and other animals; others primarily affect the microbe's spread among populations. Although it is a difficult, if not impossible, task to predict the emergence of "new" infectious diseases/agents, it is helpful to understand the factors that facilitate the emergence and spread of infectious diseases in general. We must focus on what we do know: the infectious disease that will emerge or reemerge is likely to do so through one or more of the "facilitative pathways" diagrammed in Figure 2-1. An awareness of this system of pathways constitutes the first step to reasoned prevention and control of infectious diseases.
Many of the diseases addressed in this report have emerged because of a combination of factors. This is not surprising, given the often complex interactions of microbes, their human and animal hosts, and the environment. As much as possible, however, the committee has attempted to illustrate specific causes of emergence with diseases or agents whose emergence is primarily due to that one factor.