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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Host-Parasite Relationships

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Changes in Microbial Pathogenicity

Many of the major public health crises of the past two decades have been infectious in origin.

CHANGES IN MICROBIAL PATHOGENICITY

Many of the major public health crises of the past two decades have been infectious in origin. If we examine them closely, many can be seen to be a natural consequence of hu- man behavior and progress. For example, Legionnaires’ disease can be traced to subtle differences in human behavior and social convention. Legionella pneumophila is widely found in nature as an infectious agent of predatory protozoa and is normally found in potable water supplies throughout the world. But showers and other widespread aerosolization technology (eg, spray devices for produce in supermarkets) can introduce the bacteria into the alveolus of the lung. Legionella finds a new niche in the human phagocytic macrophage instead of its usual protozoan hosts Acanthamoeba or Hartman-nella. The microbe is programmed to replicate, and the consequence is characterized as anew or emerging infectious disease. Women in our society asked for more absorbent tam-pons to achieve more social freedom and unwittingly, American commerce supplied a product that helped select for certain strains of staphylococcus. Another new emerging disease, toxic shock syndrome (TSS) was recognized and caused near panic.

These examples are not meant to turn our attention away from the pathogenic traits of the disease-causing microbes, but it seems true, on reflection, that humans, with their technology and social behavior, have played a significant role in providing pathogenic microbes with new venues for their wares. Food poisoning by Escherichia coli O157:H7, Campylobacter, and Salmonella arise as much from food technology and modern fooddistribution networks than from any fundamental change in the virulence properties of the bacteria in question. HIV, Hantavirus, and Lyme disease seem likely to be a consequence of the encroachment of humans on previously undisturbed ecological niches and the in-creased likelihood of human contact with animal species and their carried microorgan-isms. In the case of HIV, the expansion of rapid travel throughout the globe magnified this consequence. No part of our planet is more than 3 days away by air travel, a fact known and feared by all public health officials.

Today, physicians deal more and more with opportunists because our population is get-ting older, and the practice of medicine keeps individuals alive longer by surgical proce-dures and powerful drugs that affect the immune status. As a consequence, in the Western world, microorganisms that a scant 40 years ago were considered harmless commensals or environmental isolates are now feared opportunistic pathogens. Many of the primary pathogens such as measles virus are controlled now by immunization. One view is that infectious diseases are under control. Another view is that the host – parasite relationship is still in a dynamic state. Just as many people die of infection as did 40 years ago; they just die later and because of different infectious agents. It is important to understand that for most of the world, the “classic” pathogens of history such as malaria, the tubercle and lep-rosy bacillus, and the cholera vibrio, together with newcomers such as HIV, are the leading causes of human misery and death.


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