Disease has been classically described as the result of an epidemiological triad, where disease results from the interaction of the human host, an infectious agent or toxin, and the environment that promotes the exposure.1 In some instances, an arthropod vector such as a mosquito or tick is involved. Among the assumptions necessary for this interaction to take place is that there is a susceptible host. The susceptibility of the host is influenced by a variety of factors including genetic, nutritional, and immunological factors. The bacteria, viruses, fungi, and parasites responsible for disease can be transmitted in either a direct or indirect fashion (Table 8.1). Different organisms spread in different ways, and the potential of a given organism to spread and produce outbreaks depends on the characteristics of the organism and the route by which it is transmitted from person to person.
Diseases can be defined as endemic, epidemic, and pandemic. Endemic can be defined as either the habitual presence of a disease within a given geographical area, or as the usual occurrence of a given disease within such an area. Epidemic can be defined as the occurrence in a community or region of disease, clearly in excess of normal expectancy, and derived from a common source or from a propagated source. Pandemic refers to a worldwide epidemic. The usual or expected level of a disease is determined through ongoing surveillance.
Microorganisms are very efficient at infecting humans, using a number of different strategies and mechanisms. These are exemplified both by the various
TABLE 8.1 Modes of agent transmission (modified from ref. 1)
Contact (person-to-person) Indirect transmission Common vehicle Single exposure Multiple exposures Continuous exposure
Vertical (transmission from one generation to another)
strategies devised by the microbe to survive prior to infecting a host such as sporulation or harboring in drought-resistant mosquito eggs, and by the various modes of transmission, e.g., direct contact (including large droplets) or indirect contact with fomites, or by insect vectors, and airborne via small particle droplets.3 Natural experiments, however, have highlighted the true diversity in the abilities of microorganisms to infect humans and animals: Salmonella outbreaks due to contaminated alfalfa sprouts4 and to ice cream made from milk that was contaminated in a tanker that had previously contained raw eggs,5 legionellosis associated with grocery misters,6 Severe Acute Respiratory Syndrome (SARS) and Ebola hemorrhagic fever in healthcare facilities,7,8 the translocation of Rift Valley fever virus from Africa to the Arabian Peninsula and West Nile virus to the U.S.,9,10 and pneumonic tularemia on Martha's Vineyard from mowing over a rabbit.11 These few examples are a semblance of the seemingly endless list of novel ways that agents and their vectors are spread. The ability to exploit newly created biological conditions is the hallmark and challenge of emerging infections.12
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