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100,000

10,000

Figure 12.1 Infectious diseases with annual global mortalities of at least 100,000.

10,000

Figure 12.1 Infectious diseases with annual global mortalities of at least 100,000.

development of antiseptic surgical techniques in 1864 succeeded in greatly reducing the number of such often fatal outcomes. In 1873, Norwegian physician Gerhard Henrik Armauer Hansen, who oversaw a leper hospital, proposed that a specific bacterium (now called Mycobacterium leprae) was responsible for leprosy.

However, it was not until the definitive work of German Robert Koch (Figure 12.2) in 1876 that bacteria were proven to be agents of disease. About 10 years earlier, C. J. Davaine and others had shown that rod-shaped bacteria were present in the blood of animals suffering from anthrax (an important disease of animals, occasionally transmitted to humans), but not in that of healthy animals, and that an injection of infected blood would cause anthrax in previously uninfected animals. Koch provided the final proof of the bacterial etiology (causation) of anthrax by isolating the organism (now called Bacillus anthracis), growing it in pure culture in the laboratory, and injecting it into healthy mice, which then developed anthrax.

Koch was thus the first to meet the criteria for proving the causative relationship between a microorganism and a specific disease, as proposed in 1840 by German J. Henle (one of Koch's teachers). These are now referred to as Koch's postulates:

1. The microbial agent must be present in every diseased organism, but not in healthy organisms.

2. The agent must be isolated from the diseased host and grown in pure culture.

3. Inoculation of a healthy susceptible host with the pure culture must result in the same disease.

4. The same agent must be recoverable from the experimentally infected host.

These postulates, which Koch published in 1884 following his work on tuberculosis (which later won a Nobel prize), still represent an important benchmark used to judge whether there is a causal relationship between a particular microbe and a given disease.

Figure 12.2 Robert Koch. (Photo by F. H. Hancox, 1896.)

The methods that Koch developed also helped lead to rapid identification of numerous other causative agents of disease (Table 12.2).

Prevalence and Distribution of Diseases Diseases can vary widely in their prevalence

(fraction of individuals infected) and geographic distribution. Epidemiologists (scientists who study diseases and their transmission) refer to an endemic disease as one that is constantly present in a specific area, usually at a low level (relatively few affected individuals). An epidemic refers to a disease with an unusually high prevalence in a specific geographical area. A pandemic is a widespread—nearly global—epidemic. The term outbreak is used to describe a sudden increase in the prevalence of a disease in a specific population; this may be associated with a single source, such as a contaminated water supply or food. Each victim may be called a case.

Disease Transmission A disease reservoir is a site in which an infectious agent remains viable so that it can serve as a source of infection for new hosts. Commonly, this is the pool of already infected hosts, so that humans are the primary reservoir for many human diseases. However, some disease organisms can also infect other species, and some go through a complex life cycle in which the host species alternate for the various life stages.

TABLE 12.2 Chronology of Some Major Disease Agent Discoveries

Disease

Microbial Agenta

Discoverers

Year

Anthrax

Bacillus anthracis

Koch

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