The Power of Plagues

The earliest evidence of hominids, that is, animals ancestral to modern humans and not closely related to other monkeys and apes, is found in Africa. The name Homo habilis, or "handy man," is based on the fact that altered stones and animal remains have been found with the fossil bones. Researchers imagine that H. habilis lived at the edge of shallow lakes and in crude rock shelters. When the populations of H. erectus left Africa, some of their parasites went with them—but only those that could be transmitted directly from person to person. But as H. erectus encountered new environments with new kinds of animals, they were subjected to sources of new parasites; and with an increase in the number of humans living in more restricted geographical environments, the probability of large-scale infections was enhanced. Development of techniques and practices for agriculture and animal husbandry progressed step by step in sequential fashion. It may not be accidental that the first known and highly organized religions arose coincident with the agricultural revolution. Researchers have little precise information about the parasitic diseases that afflicted their ancestors more than 10,000 years ago. The crowd diseases of humans, such as smallpox, measles, pertussis (whooping cough), tuberculosis, and influenza, were initially derived from very similar ancestral infections of domesticated animals.

During the last 2,000 years, three great bubonic plague pandemics have resulted in social and economic upheavals that are unmatched by those caused by any armed conflict or any other infectious disease. The contagious nature of plague led to the belief that the only way security could be achieved was total isolation of the sick. Plague-infected rodents, now assisted by modern steamships and railways, quickly spread the disease to the rest of the world. In the three pandemics, it is estimated that rat-borne bubonic plague killed more than 200 million people. Agriculture could not keep up with the population rise, and over the next century famines occurred every few years. The result was poverty and misery, especially in the crowded and filthy cities. Deaths from plague radically reduced the average life expectancy from 35 or 40 years to 20. Medieval society had four kinds of medical practitioners: academic physicians, who knew theory but did not care for the sick; surgeons, who learned their trade as apprentices and who were the principal caregivers of the sick; barbers, who did bloodletting and minor surgery; and those who practiced folk medicine, mostly women.

Typhus fever is sometimes called “war fever” because frequently it is a companion to hostilities. During wartime, individuals are subjected to increased stress, they become more susceptible to new diseases, and endemic diseases may become more severe. Today, there is usually little in the way of epidemic disease in the industrialized countries of the world because of improvements in hygiene, but elsewhere typhus can and does surface, especially in places where there is war, famine, and poverty. The infected peasants served as a source of infection, and their lice were effective vectors. Typhus is not the same as typhoid fever, which is a water-borne disease caused by a bacillus, Salmonella. Today we know the causative agent of typhus to be Rickettsia prowazekii, related to R. rickettsii, the organism that produces Rocky Mountain spotted fever. It is human lice—including the body louse Pediculus humanus corporis and the head louse Pediculus humanus capitis, as well as the crab louse Phthirus pubis—that are involved in rickettsial transmission of typhus from human to human by means of fecal contamination of the bite wound. Typhus is not dead. It will live on for centuries, and it will continue to break into the open whenever human stupidity and brutality give it a chance, as most likely they occasionally will. But its freedom of action is being restricted and more and more it will be confined, like other savage creatures, in the zoological gardens of controlled diseases.

Smallpox was at one time one of the most devastating of all human diseases, yet no one really knows when smallpox began to infect humans. The best evidence of smallpox in humans have found in three Egyptian mummies, dating from 1570 to 1085 BC. The first outbreak of smallpox in the Americas was among African slaves on the island of Hispaniola in 1518. There are two pathologic varieties of the smallpox virus: Variola major and Variola minor. In the 17th century, smallpox was Europe's most common and devastating disease, killing an estimated 400,000 each year. With the outbreak of war in 1775, the most dangerous enemy the colonists had to fight was not the British but smallpox. Eradication of smallpox was possible for three reasons: First, there were no animal reservoirs. Second, the methods of preserving the vaccine proved to be effective. Third, the vaccine was easily administered. Smallpox is the first and only naturally occurring disease to be eradicated by human intervention. Smallpox also provided the incentive for the development of protective measures (variolation and vaccination), affected the cultural responses to disease, and contributed to the establishment of more humane public health policies. The eradication of smallpox has demonstrated that through vaccination there is the possibility for the elimination of other infectious diseases that continue to plague humankind.

Secular medicine, which developed in parallel to the religious approach, is usually dated from the time of the Greeks in the fifth century BC and most often is personified by the physician Hippocrates of Cos. Hippocrates, often regarded as the father of modern medicine, believed there was a rational basis for disease. His theory on the mechanism of disease was based on an imbalance in the body of the four humors (fluids). The majority of people were treated by the literate clergy, especially the monks (who became physicians), women who were acquainted with medicinal plants, and lay surgeons who treated wounds. By 1918, few “old guard Listerians” still believed in antiseptic surgery. At this time (and even before), surgery was an unsafe practice because control of infection (by asepsis and antisepsis) and anesthesia were unknown. Sanitation, antisepsis, asepsis, and anesthesia prevented disease from needlessly destroying human life, but perhaps the most important point of these advances in health and medicine was their influence on the social climate and the greater acceptance and recognition of scientific truths. A review of how one of the more recent antibiotics, sulfa drugs, was discovered shows how its development was linked to aniline dyes. In concert with improvements in immunization, these plague protectors—imperfect as they were and as they continue to be—benefited the health of many people, but regrettably not all of humankind.

Tuberculosis is an ancient disease that has plagued humans throughout recorded history and even before. Tuberculosis of the lungs (called pulmonary TB) is the form of the disease we are most familiar with, giving rise to the slang word “lunger". Three mycobacteria, Mycobacterium tuberculosis, M. leprae, and M. avium, are human pathogens that respectively cause TB, leprosy, and a pulmonary disease with swollen glands in the neck. Genetically, M. bovis and M. tuberculosis have been shown to be >99.5% identical, so differences in their pathogenic nature are still to be explained. The increased density of people provided ideal conditions for the aerial transmission of M. tuberculosis and pulmonary TB. Tubercle bacilli are rather robust and can survive in moist sputum for 6 to 8 months. Infection may result if as few as 5 bacteria reach the grape-like clusters of the thin-walled air sacs (alveoli) of the lung. Primary tuberculosis is a self-limiting infection that often goes unnoticed: it appears to be a cold, and usually there is little impairment of lung function. If there is a protective immune response, which occurs in 85 to 90% of cases, the disease may progress no further, calcification of the tubercles may take place, and the tuberculin test remains positive. Chest X rays show tubercles and fluid in the lung, and sounds of gurgling and slush can be heard through a stethoscope placed on the chest.

One of the most deadly African diseases is sleeping sickness. It is undoubtedly of African origin and today remains indigenous to that region. Inoculation of blood containing trypanosomes into healthy animals produced surra. Trypanosoma gambiense gives rise to a mild chronic infection found in western and central Africa and is transmitted by riverine species of Glossina (G. palpalis and G. tachinoides) that are associated with human habitation. Asymptomatic individuals may harbor parasites in the blood for long periods of time and could be a source of infection for the vector. The disease is an anthroponosis: fly, to human, to fly, to human. T. rhodesiense is the East African form and is the more virulent subspecies, producing an acute infection. The spread of sleeping sickness was increased about 1890 because of another infection brought into Africa by cattle from outside the continent: rinderpest, a virus of cattle and game with high mortality. Treatment of sleeping sickness is of limited value and depends on early use of rather toxic drugs administered intravenously. Pentamidine, melarsopol, and suramin are all used. River blindness is transmitted by a blackfly called Simulium damnosum, which carries the microscopic infective juvenile stages (called microfilaria) of Onchocerca. The economic and social consequences of river blindness are profound, and surely much of the local history is tied to the progressive depletion of human resources.

Many factors lead to the emergence of infectious diseases, some of which are obvious: a change in a parasite’s virulence, a breakdown in public health and surveillance measures, a change in human behavior, crowding, alterations in the environment, technological advances, economic downturns, poverty, and even something as innocent as foreign travel for a vacation. The disease is more prevalent in men than in women, probably as a result of the greater occupational exposure of men to rodents. Human-to-human transmission did not occur, but one wonders what would have been the result if the virus had acquired the capacity for pulmonary transmission, as happened with pneumonic plague. West Nile virus (WNV) was first isolated in 1937 from the blood of an infected woman in the West Nile country of Uganda. The woman had a rapid onset of fever, headache, backache, muscle pains, and anorexia. The most significant factor in all of the outbreaks was the involvement of the common house mosquito Culex pipiens as the vector. It is likely that WNV will advance through the United States within the next several years, and its path of death will be marked by the local and regional flyway patterns of virus-infected birds. Although sheep farmers tended to believe that scrapie was a hereditary disease, there was now mounting evidence that transmission was by sheep eating the afterbirth of an infected animal or grazing in a pasture where infected animals had been kept.