The Power of Plagues. Irwin W. Sherman

in the process wool barons. The smaller labor force in the cities led to their having a stronger negotiating position, and so there was a need to pay higher wages; as a consequence, there was an improvement in the standard of living. Many individuals, however, fled the cities and moved to the country, and there the survivors were induced by landowners to harvest the crops by the provision of higher wages. The standard of living among the urban and rural populations improved, and the laboring classes became more mobile. The Black Death was thus a contributing factor in altering the social and economic structure in England, and by the 16th century landlord and serf ceased to exist, although feudalism did linger for several more centuries in continental Europe.

Governments that sought to control wages sowed the seeds of discontent both in the city and on the farm. This led to what is called factor substitution—cheap land and capital were substituted for the more expensive labor. Peasants would not accept a lease unless the landlord provided additional capital in the form of oxen and seeds. In the city, tools and machines (capital) were substituted for human labor. As plague continued to deplete the numbers of guild members as well as the pool from which new members could be drawn, the guilds had to take on new apprentices without family connections to a particular trade. Shorter years of apprenticeship, rapid turnover in guild members, and increased recruitment of new members led to a decline in the quality of the product. Sometimes the higher labor costs led to technological innovation in the form of labor-saving devices. One such was the invention of movable type and the printing press. Scribes who had been employed to copy sheets of manuscript and then to assemble these into a bound volume could not keep up with the demand for their services with an emerging and increasingly more literate population; a way out of this labor-intensive method of bookmaking was to find a cheaper solution. Johannes Gutenberg’s invention in 1453 of printing using movable metal type was one way that this was accomplished.

      Another was to employ an economy of scale in sea- and oceangoing transport vessels. Bigger ships with smaller crews could remain at sea for longer periods of time and would be able to sail directly from port to port, but this would require better ship construction, improvements in navigational instruments, and new business enterprises such as maritime insurance to protect the investment in cargo and the ship. As a consequence, merchants such as bankers and craftsmen became more powerful. The new economy became more diversified, there was a more intensive use of capital, technological innovations became more and more important, and there was a greater redistribution of wealth. In time the aristocracy found it had to yield power to the masses. The social and economic fabric of Europe began to be altered, and it was the Black Death that instigated such change.

      Finding the Killer

      As plague raged through medieval Europe, it became increasingly apparent that this disease was contagious. Even if Fracastoro’s idea of “seeds of contagion” was accepted, however, at this time there would still be no means to identify precisely the causative agent. Identifying the “seed” would not only require a technological innovation; it would also require a change in the concept of infectious (contagious) diseases. Three centuries after Fracastoro’s theory of contagion, the concept that disease could result from the invasion of the body by microbes or germs—the germ theory of disease—was established. And with a 17th-century technological innovation, the microscope, it was actually possible for humans to see germs! Two schools of thought, one in France under the leadership of Louis Pasteur (1882-1895) and the other in Germany, led by Robert Koch (1843-1910), were responsible for firmly grounding the germ theory, and for all of their lives these two microbe hunters remained fierce competitors (see p. 418).

As plague ravaged China during the third pandemic, Pasteur dispatched Alexandre Yersin (1863-1943), a Swiss-born member of the French medical colonial corps, to Hong Kong to study and attempt to isolate the germ of plague. Yersin arrived when the epidemic was in full force and was given a small table in a dark corridor next to a patients’ room, where he could leave his microscope, notebook, stains, pipettes, and a few cages with guinea pigs, mice, and rats. At first Yersin was not permitted access to the morgue, but through connivance and bribery he was finally able to visit the morgue for a few minutes to examine a sailor who had just died from plague. Yersin punctured the bubo on the dead soldier’s thigh (Fig. 4.6A) with a sterile needle and removed some fluid; he then examined the fluid under the microscope, inoculated a few guinea pigs, and sent the remainder to the Pasteur Institute in Paris. On June 24, 1894, he wrote Pasteur that the fluid was full of rod-shaped bacilli (Fig. 4.6B) that stained poorly with Gram stain; i.e., they were Gram negative. (Gram stain consists of a tincture of crystal violet. After bacteria are killed and washed, some lose the dye whereas others retain the purple color; the former are called Gram-negative and the latter Gram-positive bacilli.) Yersin also wrote: “without question this is the microbe of plague.” A few days later Yersin found that the guinea pigs he had injected with the fluid that he had aspirated from the bubo had died, and that their bodies swarmed with the same bacilli. Yersin was intrigued by the large number of dead rats in the streets of Hong Kong, as well as in the hospital corridors and the morgue. When he examined some of these dead rats, he found the same bacilli to be present. He correctly concluded that plague infects both rats and humans. At about the same time, Koch, convinced that plague was caused by a microbe, sent his associate Shibasaburo Kitasato and a large number of assistants as well as abundant equipment to find the plague germ. Kitasato did culture a bacterium from the finger of a sailor who had died from plague, but it was Gram positive. Further, Kitasato was never able to prove that this bacillus could produce plague in humans or other animals. Bubonic plague, caused by a rod-shaped, Gram-negative bacterium, was named Yersinia pestis after its discoverer, Alexandre Yersin (although Pasteurella pestis was the name used before 1970).

Figure 4.6 A. Bubo of bubonic plague (courtesy of CDC, 1993) and B. the causative agent, Yersinia pestis, stained and seen with light microscope (courtesy CDC)

      Finding the Vector

Yersin discovered the plague microbe, but he did not find the means whereby the disease could be transmitted. He asked himself: was it airborne, or did it have to do with rat feces or dried rat urine? The answer to the question came from Paul-Louis Simond (1858-1947), a French army physician who was sent by Pasteur to Vietnam and India to follow up on Yersin’s observations. Simond noted that not only were there large numbers of dead and dying rats in the streets and buildings, but that 20 laborers in a wool factory who had been cleaning the floor of dead rats had died of plague, while none of the other factory workers who had no contact with rats became ill. He began to suspect that there must be an intermediary between a dead rat and a human, and that the intermediary might be the rat flea Xenopsylla cheopis (Fig. 4.7). Simond found that healthy rats groomed themselves and had few fleas, but that sick rats unable to groom their fur had many; and when the rats died, the fleas moved off onto other healthy rats or onto humans. To prove this he did an experiment: a sick rat was placed at the bottom of a jar, and above this he suspended a healthy rat in a wire mesh cage; although the healthy rat had no direct contact with the plague-infected one, it did become infected by exposure to its fleas (which he determined could jump as high as 4 in. without any difficulty). As a control Simond placed a sick rat without fleas together with healthy rats in a jar. None of the healthy ones became sick, but when he introduced fleas into the jar, the healthy rats developed plague and died. On June 2, 1898, he wrote Pasteur that the problem of plague transmission had been solved.

Figure