Although the vast majority of microbes pose no threat whatsoever to human health, there are many forms that are outright hazards. The importance of the infectious diseases they cause is demonstrated by the rates of mortality (death) to which they can be linked. Even today, infectious disease is the world's leading cause of death, with fatalities exceeding 15 million per year (Figure 12.1). Rates of overall morbidity (illness, both fatal and nonfatal) are of course much higher and have a tremendous impact on the world's economy and each person's quality of life.
Respiratory and gastrointestinal diseases account for over half of these deaths. It is also sad to note that the majority of these deaths are of children below the age of 5 (the criterion used for infant mortality). Underdeveloped or developing countries are most heavily hit. Within industrialized countries such as the United States, for instance, the infant mortality rates fall below 1%, but these figures skyrocket to nearly 10% in developing countries and to more than 15% in the world's least developed countries. Furthermore, about half of the world's population is considered to be at risk to a wide range of infectious diseases.
Indeed, within the past millennia, microbial disease has proven to be a formidable adversary, one that has the potential to decimate the human population if left unchecked. During the Middle Ages and extending into the nineteenth century, diseases such as bubonic plague, cholera, and typhoid swept through Europe, causing massive mortality. The influenza pandemic at the end of World War I, for example, killed more people than the war itself.
Microorganisms are ubiquitous on and within the bodies of virtually all higher life-forms (excluding those raised in laboratories under special "germ-free" conditions), and for the most part they are innocuous for their host (the organism that supports them). However, abnormal proliferation of indigenous microbes, or invasion from an external source, can lead to disease. Most of the important problems are infectious in nature, being spread by the dissemination of viable, pathogenic cells from one host to another by either direct or indirect means. In many instances, pathogenic microbial agents may also subsist within an environmental reservoir, lingering in wait for an opportunity to assert their influence.
A parasite is an organism that lives in a close relationship with another organism, benefiting at the expense of its host. Pathogens are thus parasites that do enough harm to their host to result in disease. However, it is also common to call disease-producing viruses, bacteria, and fungi pathogens, while referring to infective protozoans and worms as parasites.
TABLE 12.1 Examples of Normal Microbial Biota of Human Body Regions3
Nose, nasopharynx, and sinuses
Mouth and throat
Lower respiratory tract Stomach (pH ~ 2)
Acinetobacter Corynebacterium Propionibacterium acnes Staphylococcus aureus Staphylococcus epidermidis Streptococcus Haemophilus Neisseria
Staphylococcus aureus Staphylococcus epidermidis Streptococcus pneumoniae Streptococcus pyogenes Corynebacterium Fusobacterium Neisseria
Staphylococcus epidermidis Streptococcus mitis S. salivarius Treponema
Normally, few microorganisms
Streptococcus aAll are bacteria, except Candida, a fungus.
The Germ Theory of Disease and Koch's Postulates We now take for granted that infectious diseases are caused by microbes, but this is actually a relatively new concept. Although Leeuwenhoek (Section 10.2.1) had first observed microbes in the seventeenth century, they were generally thought to be too small and unimportant to affect the health of higher organisms. In the early nineteenth century, fungal diseases of plants and later animals (silkworms) were first recognized. When Pasteur (Section 10.2.2) summarized his findings on the germ theory of disease in 1862, and referred to microbially caused spoilage as "diseases" of wine and beer, this influenced Englishman Joseph Lister to theorize that surgical wound infections might be the result of bacterial growth. His
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