Germs, Genes, & Civilization: How Epidemics Shaped Who We Are Today

by David P. Clark

  Cholera and good intentions

  Moralizers often blame the spread of epidemic disease on imperialism and exploitation. However, neither infectious agents nor their insect vectors subscribe to human ethics, and the spread of infections is neither hastened nor hindered by the moral (or immoral) intentions of the humans who carry them. Consider the spread of cholera. During the first worldwide cholera pandemic in the early nineteenth century, British ships carried cholera, which originated in India, to Muscat in Arabia. The British landed an expeditionary force there in 1821 to suppress slavery. From Muscat, cholera spread to the East Coast of Africa with the retreating slave traders. It then spread to the Persian Gulf and much of the Middle East.

  Most contact between groups of people is the result of trading rather than the two extremes of invasion or charity. Regardless of human motivation, the spread of viruses such as smallpox, measles, and influenza merely requires close human contact. If Cortez had sailed to America to trade with the Aztecs, he would still have carried smallpox, measles, and influenza. If an international charity had sent Cortez to provide aid, the viruses would have gone, too. The overall demographic results would have been much the same in all cases.

  The issue of biological isolation

  Various authors have suggested that typhus and tuberculosis were present in both the New World and the Old World before contact. If so, the ancestors of these infectious agents must have crossed the Bering Strait with migrating humans around 10,000 years ago and then been isolated along with their human hosts. This generates a major biological paradox because microorganisms evolve rapidly. After 10,000 years of isolation, the New World and Old World descendents of any microorganism should have differed significantly in DNA sequence—and probably in clinical symptoms, too. Indeed, the Old World and New World species of trypanosome that cause sleeping sickness and Chagas disease, respectively, are quite distinct. Trypanosomes are protozoan parasites that evolve much more slowly than the bacteria responsible for typhus and tuberculosis. From an evolutionary perspective, it is extremely unlikely that descendents of such bacteria could remain indistinguishable if isolated for 10,000 years on different continents.

  Such biological considerations imply that typhus and tuberculosis might have been present before contact in either the New World or the Old World, but not both. If the continents were truly isolated, we are therefore forced to choose one homeland for each of these infections. But just how isolated were the Eurasian and American continents from the perspective of infectious microorganisms?

  Flocks of birds routinely migrate across the Atlantic Ocean. The microbiological consequences of this were recently illustrated by the appearance of West Nile Virus in New York in 1999. West Nile Virus infects a wide range of birds, among which it is spread by blood-sucking insects. Within the United States, it is spread by mosquitoes. The virus is widely distributed in Africa and the Middle East, but it had not previously been seen in the Americas. Birds carried the virus across the Atlantic, and DNA analysis suggests an origin among domestic geese in Israel. The issue is complicated by the modern possibility that birds sometimes rest briefly on ships, and insects can be carried accidentally by aircraft.

  Spotted fevers and rickettsias

  According to the records of the Aztecs and Incas, spotted fevers of some sort were present in precontact America. Degenerate bacteria known as rickettsias cause this group of closely related diseases. Rickettsias are much smaller than typical bacteria. They have degenerated and become so dependent on the host cells they infect that they cannot be grown in culture as normal bacteria can. (Note that the bone disease rickets has nothing to do with rickettsias. Rickets results from vitamin D deficiency, not infection.)

  Rickettsias are spread by lice, fleas, or ticks. They are primarily infections of animals, transmitted to humans when their carriers jump from animals to nearby humans and bite them. Two major groups of rickettsial spotted fevers exist, each containing several closely related infections that are difficult to tell apart clinically because all cause spots and fever. The most famous is typhus, best known for devastating Napoleon’s armies. The typhus fevers are generally regarded as Old World diseases, whereas the other spotted fevers, such as Rocky Mountain spotted fever, are of New World origin. However, things are not as clear-cut as they might seem.

  Murine typhus primarily infects mice and other rodents. It is spread by fleas and lice, which can pass on murine typhus to humans by jumping or crawling from rodent to man. Epidemic typhus is a derivative that has adapted to humans. It is carried by human lice. Sophisticated diseases (such as malaria and yellow fever) do not usually kill their insect vectors. Similarly, murine typhus does not kill the fleas or lice that transfer it. In contrast, epidemic typhus does kill human lice, suggesting that epidemic typhus is a relatively novel variant, not yet fully adapted to its carriers.

  The origins of typhus are uncertain

  Although earlier reports indicate epidemics that might or might not have been typhus, the first unambiguous outbreaks were in the early 1500s, when the French and Spanish were fighting for control of Italy. From the beginning, typhus was associated with military campaigns and seems to have taken a clear dislike to the French, in particular. In 1528, the French were forced to withdraw from Naples after losing 30,000 men to typhus, which they presumably caught from the Spanish. From a focus in Spain, typhus spread around Europe. According to Spanish author Joaquin Villalba, the typhus epidemic of 1557–1570 depopulated much of the Iberian Peninsula. The timing and location supports the view that typhus might have been a New World import.

  Supposed outbreaks of “typhus” in the 1400s in Germany or France are sometimes mentioned. In England, an outbreak of jail fever in 1444 killed 5 jailers and 64 prisoners. In recent centuries, “jail fever” has come to mean some version of typhus. However, back in the good old days, before different diseases were distinguished, jail fever was just that—what people caught in jail.

  The Chinese medical classic the Zhouhou Beijifang lists a variety of diseases present in the Far East in the fourth century A.D. Among these is “Japanese river fever” or “sand-lice disease,” often identified by modern commentators as tsutsumagoshi fever, or scrub typhus. The symptoms given are fever, bodily aching, and a rash. Several diseases fit these symptoms, but, if anything, the severe aching suggests dengue fever, which is widespread in the Far East and spread by mosquitoes.

  In 1576, typhus, referred to as cocoliztli, swept Mexico, killing an estimated two million people. Supposedly, the Spanish rarely died. This does not square well with Villalba’s claim that the 1557–1570 typhus epidemic depopulated the Iberian Peninsula. Nor does the higher death rate among the Aztec population support an American origin for this disease. However, we should remember that typhus is typically a disease of colder climates that relies on dirty clothing to harbor the lice that carry it. If typhus did come from America, it probably came from the colder, mountainous Inca lands instead of Central America. Thus, for the people of Mexico, typhus might well have been a new disease, brought back from Peru by the Spanish.

  Although we cannot be sure, it seems likely that typhus and the related spotted fevers originated in the New World. Upon contact, typhus traveled back to Europe with the Spanish. In Europe, it increased in virulence over the next couple centuries as warfare provided it with a convenient means of transmission. Although more diseases have traveled from the Old World to the New, typhus and syphilis (see Chapter 7, “Venereal Disease and Sexual Behavior”) appear to have been acquired in exchange. Admittedly, this is hardly fair trade, but virulent infections did not all travel in one direction, as is often suggested.

  What about the Vikings?

  In reality, Columbus was not first to discover America. The Vikings beat him to it. In 981, Eiríkr Thorvaldsson, better known as Eric the Red, explored Greenland. Eric’s father was banished from Norway and went to live in Iceland. Eric himself was banished from Iceland and so set sail westward to explore. He found Greenl
and. After returning to Iceland, he led an expedition of 25 ships to Greenland because it had better grazing land than Iceland. Eric the Red apparently named the island Greenland to attract settlers. From Greenland, Leif the Lucky, son of Eric the Red, sailed to Newfoundland on the American mainland. The Vikings referred to the mainland as Vinland, and they settled there for about three years before they were driven away by the American Indians, whom they referred to as “Skrælings.” Although Vinland was visited from Greenland several more times, no further settlement was attempted.

  Notice that the Skrælings were not annihilated by diseases that the Vikings brought from Europe. This contrasts sharply with what happened when the Aztecs met the Spanish. Does this imply that smallpox, measles, and influenza were not current in Europe during this period of history? An alternative explanation is that the population of Iceland, the staging post for these voyages, was too small to maintain such diseases in circulation. Any epidemics brought from the European mainland to Iceland burned themselves out. Hence, the Vikings were relatively germ-free when they set sail and consequently failed to wipe out the Skrælings and colonize North America.

  Although it seems clear that the Vikings did not bring any spectacular epidemic virus diseases to America, they may have brought other, less dramatic infections. Did they carry typhus? Typhus is spread by lice that inhabit human hair and clothes. Dirty clothes that are not changed or washed for the duration of an ocean voyage are ideal for lice to live and lay their eggs in. Because fur coats provide better homes for lice than loincloths, typhus is typically a disease of the colder regions. If typhus was extant in Europe at the time, it is hard to imagine the Vikings failing to take it with them. Here, then, we have an alternative scenario for typhus as a European disease that the Vikings carried to America before the Spanish invasion.

  The Viking settlement in Greenland survived until the second half of the fourteenth century. Planetary cooling during this period made Greenland progressively colder and less fertile. But the final blow was plague. The Black Death of the mid–fourteenth century killed around 50% of the people of Scandinavia and Iceland. From there, it hit Greenland, probably in the winter of 1350. The details are unknown, but when the Norwegians visited Greenland in the early 15th century, they found only deserted villages. Presumably, the Greenlanders were wiped out before they could pass the Black Death on to the Skrælings.

  10. Urbanization and democracy

  Cities as population sinks

  The growth of urban civilization conceals a major paradox. Clearly, the formation of the city-states of the Middle East and ancient Greece is good evidence for increasing population. The urbanization of Western Europe was also accompanied by an overall increase in population. What is rarely appreciated is that, until relatively late in the industrial era, the death rate in the cities was higher than the birth rate. Consequently, the populations of medieval cities were replenished by migration from the countryside. During most major periods of city growth, the population increases were actually produced in rural areas; the cities themselves had a net negative impact on the population.

  The death toll in the towns was largely the result of infectious disease. Infant mortality was often greater than 50%, and many of those who survived infancy failed to reach adulthood. In medieval Europe, overall life expectancy ranged from the mid-twenties to the upper-thirties. As industrialization proceeded, life expectancy rose and infant mortality fell. However, even in England, it wasn’t until the eighteenth century that most towns no longer needed rural immigrants to maintain their numbers. Rural areas in medieval times were healthy only in the sense that they generated a net increase in population. Infant mortality and life expectancy were not as abysmal as in the towns, but they still were horrific by modern standards. Many—perhaps most—of the peasants who survived infancy died of fungal infections of the lungs, caught from spores infecting their crops. In contrast, the towns were home to the bacterial and viral diseases with which our modern city-oriented culture is more familiar.

  Viral diseases in the city

  The growth of dense populations and their crowding into towns and cities allowed the emergence of highly contagious viral diseases that circulate only among humans, such as smallpox, measles, mumps, and rubella (or “German measles”). By industrial times, most of these had become childhood diseases, causing very few deaths. Even smallpox, though still a significant cause of death, had become much less dangerous. In 735–737, smallpox killed around 75% of those infected and annihilated half the population of Japan. By the late nineteenth century, the death rate from smallpox itself (Variola major) was around 20% of those infected, and a variant known as alastrim, or Variola minor, with a death rate of only 1% to 2%, was spreading. Infection with V. minor creates immunity to the more dangerous type of smallpox, and it seems likely that if nature had been left to take its course, in a century or two, V. minor would have displaced V. major, relegating smallpox to a childhood disease comparable to measles. What actually happened was that human intervention led to the eradication of smallpox by vaccination. The last cases of natural smallpox occurred in Bangladesh, in 1975 for V. major and 1977 for V. minor.

  Bacterial diseases in the city

  Bacteria do not evolve as fast as viruses, so most new bacterial diseases of humans are still shared with other animals. We can divide the bacterial diseases of the growing urban populations into three main groups: those spread directly from person to person, those spread by dirty water, and those spread by insects. Bacterial diseases spread from person to person, usually by droplets coughed or sneezed into the air, were common in towns and cities until recent times. Examples include scarlet fever, diphtheria, whooping cough, and tuberculosis. These diseases spread in a manner similar to that of measles and smallpox, although less efficiently. Vaccination has largely eradicated most of these diseases from advanced nations.

  We saw in Chapter 4, “Waters, Sewers, and Empires,” how the collapse of the Indus Valley civilization was probably linked to cholera or a similar waterborne infection. Typhoid, dysentery, and a host of lesser diarrhea-causing diseases took a steady toll throughout the period following the Roman Empire, especially in crowded communities whose water supplies were at risk of contamination. As technology advanced, these diseases have gradually faded away in industrialized nations. Today various diarrhea-causing strains of Salmonella and E. coli have re-emerged as public health hazards, less in the water supply than in batches of contaminated food, especially processed meat.

  The Black Death

  The outstanding example of a bacterial disease spread by insects is the Black Death, or bubonic plague. In the middle of the fourteenth century, it wiped out around half the population of Europe. Very likely it did as much damage in the more densely populated parts of Asia, the Middle East, and North Africa, although detailed records are available only for Europe. Lesser outbreaks of bubonic plague reverberated around Europe until the 17th century.

  Bubonic plague is caused by the bacterium Yersinia pestis, which infects many animals, especially rodents. From these, it can be transmitted to humans (and their cats and dogs) by fleas. Why would fleas leave the shelter of the densely packed hairs on furry animals such as rats to venture onto the exposed surface of relatively hairless creatures such as humans? Not through choice. Fleas come in distinct varieties and tend to stay with the animal they are adapted to. But if the animal dies, the flea can no longer obtain its required diet of fresh blood and must find a new host. So when rats or mice die of plague, their fleas leave and look for new animals to infest. Rat fleas cannot actually survive for long on humans—our blood doesn’t supply the correct balance of nutrients. But one bite is enough to transmit plague. That the flea will eventually die due to improper nutrition is small consolation.

  In nature, the plague bacterium infects wild rodents such as the marmots and susliks of central Asia, or their relatives, the ground squirrels and chipmunks, of North America. It causes only mild symptoms, often no worse than
a bad cold would be to humans. When displaced from their normal environment, the wild rodents can transfer bubonic plague to the rats and mice who live in close contact with humans. Unlike their wild relatives, domestic rats and mice fall seriously ill and are killed by plague. Their resident fleas then look for new animals to live on. This is what sets in motion epidemics of plague. The great Black Death epidemic of the Middle Ages was probably the result of climatic fluctuations in the northern Asian steppes. A few good years—for rodents—followed by a couple bad years resulted in a large rodent population with no food. So, several rodent species extended their ranges southward. This brought them into contact with other, more southern, rodents that, in turn, made contact with the human societies of Asia. The fleas and Yersinia pestis bacteria were passed along, too.

  Climatic changes: the “Little Ice Age”