Over the long term, novel diseases may adapt to humans or may go extinct. Adaptation does not imply that the disease becomes mild—merely that it gains the ability to survive and multiply in humans. The level of virulence acquired depends on the mode of transmission and how plentiful, crowded, and unhygienic the human hosts are.
Measles and its relatives
Among Old World populations, measles is a relatively mild childhood disease, only rarely causing severe complications. It is transmitted from person to person, and once a person has measles, he becomes immune and the virus disappears from his body. For measles virus to stay in circulation, it must constantly find a new supply of victims to infect. In societies where essentially everyone catches, survives, and becomes immune to measles as children, the only new hosts for measles are newborn children.
Calculations indicate that a human population of 300,000–500,000 individuals in frequent contact with each other is necessary to provide new children at a sufficient rate to prevent measles from going extinct. Before approximately 1,000–500 B.C., there were no individual cities with populations over a quarter of a million. The first to appear were Babylon, capital of the Babylonian Empire, and Ninevah, capital of the Assyrian Empire, followed shortly by Europe’s first real city, Athens. Until these were available, measles could not have been easily maintained as a human disease.
Before this period, diseases such as measles had to rely on moving from town to village within a region of civilization linked by road or river. Dense enough populations were found in the Middle East, in the valleys of the Tigris and Euphrates rivers (Mesopotamia), starting around 3,000 B.C., and later in the valleys of the Nile (Egypt), the Indus (North India and Pakistan), and the Yellow River (Northern China). Each of these areas had enough people to keep an epidemic disease in circulation, as long as the separate communities were in efficient contact. This varied considerably over the centuries, and no doubt the spread of infectious disease fluctuated wildly as a result. Many diseases probably jumped from animals to man during this early period, burned their way through a few unfortunate towns in close contact, but then failed to spread any further. Some animal diseases may have jumped several times before becoming established in the human population.
Where did measles come from? Probably from man’s best friend, the dog. Measles is a member of the distemper virus family, the Morbilliviruses. Distemper viruses are typically found in carnivores and include seal distemper and dolphin distemper, in addition to canine distemper. Canine distemper can also infect other carnivores and is found among sea lions and hyenas. Humans probably caught measles from dogs a couple thousand years ago, and it has since evolved into a milder form.
Comparing DNA sequences shows that rinderpest of cattle is the closest present-day relative of measles. However, rinderpest is an extremely virulent disease that still causes mass mortality among cattle and related wild animals (buffalo, antelopes, giraffes, wildebeest). Viruses that spread directly from person to person, or between individuals of closely related species, usually evolve to lower virulence over time (as you will see in Chapter 3, “Transmission, Overcrowding, and Virulence”). Taking this into account, the most likely scenario is that dogs passed distemper to humans, where it evolved into measles. More recently, humans passed the virus to cattle, where it evolved into rinderpest but has not yet had time to drop significantly in virulence. In agreement with this scenario, the major rinderpest outbreaks in the 1990s showed lower death rates in domestic cattle but still caused 50% or more mortality in buffalo, kudu, and other game animals. This argues that rinderpest has partly adapted to domestic cattle and has spread from there to wild game animals, where it is still highly virulent.
Measles in ancient history
Can we pinpoint the origin of measles more accurately? Not really, but we can amuse ourselves guessing. In the Iliad, Homer describes how Apollo, who favored the Trojans, sent a plague on the Greek army besieging Troy. Although Apollo was the archer god, he was not the god of war; his arrows carried pestilence. He was also the god of medicine—what gods send, they can also remove, if asked politely.
Then he sat down apart from the ships and let fly an arrow: terrible was the twang of the silver bow. The mules he assailed first and the swift dogs, but then on the men themselves he let fly his stinging shafts, and struck; and constantly the pyres of the dead burned thick.
The Iliad was written around 700–800 B.C., several hundred years after the actual siege of Troy. In his commentary written some 400 years later still, Aristotle was puzzled why Apollo would attack the mules first. Knowing as we do today that canine distemper is closely related to both measles and rinderpest, we might argue that Homer reported what actually happened: an outbreak of an early distemper virus still shared by dogs, mules, and humans. If so, the Greeks probably caught it from the more densely populated city of Troy.
As human population density increased over the next few centuries, a strain of this shared morbillivirus may have adapted solely to humans. Voilà, measles! It is even conceivable that the Great Plague of Athens (430 B.C.) was caused by the ancestral, highly virulent form of measles. At that time, Athens had just become the only region of Europe densely populated enough to maintain a disease such as measles. (At this time, the city of Athens is estimated to have had around 250,000 people and Attica, the surrounding area, another 200,000 or so.)
The morbilliviruses include several other mild diseases, including mumps and the human parainfluenza viruses. As you might guess, parainfluenza viruses cause symptoms similar to but less severe than genuine influenza. Close relatives of these are known in birds, pigs, and monkeys. Take your choice. Successive waves of infection by a series of related viruses probably entered the human population from various animals. Those that were originally lethal have by now become milder, and their symptoms are so vague that only mumps still merits a name of its own.
Diseases from rodents
Rats and mice bear the same relationship to sheep and cows as weeds bear to wheat and maize. They are neither truly wild nor truly domestic. Even though we never intended to domesticate them, our rodent pests depend as much on humans for their homes and food as animals we have deliberately tamed. Rats live in attics, basements, sewers, storm drains, and any other underground tunnels. House mice live in cavities within walls, and field mice live in barns, haystacks, and cornfields. Rodents scavenge leftovers and steal grain, cheese, and other stored foods. Though rarely seen, rats usually outnumber people in a typical human city.
The mouse was one of the earliest domestic animals. The Natufian culture, found in the Middle East about 12,000 years ago, was transitional between hunter-gatherer and a settled agricultural way of life. The remains of domestic mice were found when excavating the Natufian layer at Jericho. Presumably, mice were attracted to the stored grain and moved in with the humans. Today rats and mice star in cartoon movies, and the most famous American of all time is a mouse whose only close competitors are a duck and a rabbit. In historical times, rodents were more often star performers in spreading pestilence.
Some diseases are spread directly by rodents, but more often insects act as intermediaries. The most famous example is the Black Death (bubonic plague), spread by fleas carried by rats. This is not a uniquely human disease, but is shared by many animals.
Typhus fever is carried by lice or fleas. Epidemic typhus has adapted for human-to-human transmission via the human body louse. It is closely related to murine typhus, which spreads among rodents or from rodents to humans via fleas. The rodent disease is the ancestor of the more specialized and more virulent epidemic typhus. This specialized human version of typhus has been responsible for massive casualties among soldiers during military campaigns and among the inmates of prisons and other institutions. Most of Napoleon’s casualties during his disastrous Russian campaign fell prey to typhus. Where lice are free to roam over blankets, clothes, and hair, typhus is not long in following.
Most diseases spread by rats and m
ice are actually carried by the fleas, lice, or ticks that live on the rodents. However, several cases of direct transmission are noteworthy. Some of the emerging viral diseases, such as Hantavirus and Lassa fever, are spread by contact with rodent droppings or urine. These are both primarily diseases of wild rodents, not house mice or city rats. Unless they establish themselves in the domestic rodent population, they are unlikely to become a serious health problem. For now, as with Ebolavirus, they are a scary but rare threat, lurking on the fringes of civilization.
Leprosy is a relatively new disease
Most people are under the impression that tuberculosis and leprosy, both the result of infection by Mycobacteria, are ancient scourges of the human race. This may be true for tuberculosis, but leprosy is a much younger disease and probably came from mice. Both diseases are spread by direct personal contact, so both are under evolutionary pressure to adapt by becoming milder. Ancient traditions depict leprosy as highly contagious and virulent, hence the need to segregate lepers. In apparent disagreement with this, leprosy seems rather difficult to catch nowadays, even with prolonged exposure. Only about 10% of the family members of a leprosy victim catch the disease, despite being in close contact and also being genetically related.
Before dismissing the earlier views, let’s consider them from an evolutionary perspective. Leprosy probably was more contagious in the past. Over time, many people sensitive to leprosy have undoubtedly died, so surviving humans are inherently more resistant. In addition, to promote its own distribution, the leprosy bacterium has probably become less virulent. Although leprosy is eventually fatal if untreated, this takes a long time, thus giving the bacteria longer to find a new host.
The disease known today as leprosy is caused by Mycobacterium leprae. It is sometimes called Hansen’s disease, not merely to reduce the social stigma to its victims, but because assorted diseases causing skin lesions and disfigurement were lumped together with leprosy until fairly recently. True leprosy causes characteristic bone loss at the extremities and can be diagnosed unambiguously in skeletons dug up by archeologists. The earliest traces of leprosy found so far date to the second century B.C. in skeletons from an oasis in the Sinai desert. Thus, the leprosy mentioned in the Old Testament is not the same as our modern disease.
Where did modern leprosy come from? The culprit is probably the mouse, which harbors a severe disease caused by the related bacterium, Mycobacterium lepraemurium. Cats also suffer from a form of leprosy, although the disease rarely penetrates the internal organs and the skin lesions generally clear up naturally after a few months. Although mice get a much nastier disease, both cat and mouse leprosy are caused by the same bacterium. We may guess that around 200 B.C., M. lepraemurium made the jump into humans and then gradually degenerated into Mycobacterium leprae, which can no longer grow outside a living victim or reinfect mice.
What goes around comes around
Many human diseases, both infectious and noninfectious, are self-inflicted wounds. The present prevalence of obesity in the West is the result of overeating and underexercising. Cigarettes are responsible for lung cancer. In addition, most of our present infectious diseases are related in some way to the rise of human civilization and the domestication of livestock and, though unwanted, of rodents. In the next chapter, we ponder the spread of disease and the changes in virulence, much of which can also be blamed on human activity.
3. Transmission, overcrowding, and virulence
Virulence and the spread of disease
How a disease spreads greatly affects its impact on human society. Diseases that spread efficiently will clearly infect more people. Less obvious, but no less important, its transfer mechanism determines how virulent a disease may become.
We must tackle two widespread misconceptions. Both generalizations are half true, and scientific investigations have only recently discovered their flaws. The first is the idea that because diseases adapt to their hosts, they will inevitably become milder if we just wait long enough. Thus, syphilis was extremely virulent when first introduced into Europe but nowadays is much milder. Similarly, childhood diseases such as measles and mumps rarely do much real damage, although they were once much nastier. But this trend is not inevitable. Recent findings indicate that, under some circumstances, diseases change little in their virulence or even get worse. Moreover, some, like bubonic plague, appear to oscillate in virulence.
The second issue is the prevalence of infectious disease throughout history. We tend to think that the farther back we go in history, the dirtier and less hygienic people were, and so the higher the level of infectious disease. This is broadly true if we restrict ourselves to the last 1,000 years of Western civilization. However, if we consider the broader sweep of human history, the prevalence of infectious disease has fluctuated wildly. For example, only in the nineteenth century did Western civilization regain the level of hygiene that existed during the prime of the Roman Empire. Again, in very early times, before urbanization began, when humans were still few and far between, infectious disease was probably much less frequent.
Infectious and noninfectious disease
To understand how disease has affected our history, we must understand how infections are spread. Until recently, infectious diseases were lumped together with a variety of other ailments. Historical societies were often confused about their causes and, consequently, about what precautions to take to avoid them.
We can classify diseases according to how they are acquired. Wounds, bruises, and broken limbs are the result of accidents or deliberate violence. Ancient societies were well aware of the effects of a sword-thrust or a fall off a cliff. As with violence, poisoning can be deliberate or accidental. Early cultures certainly understood the idea of deliberate poisoning, although the victims were often misdiagnosed. For example, the symptoms of arsenic poisoning include vomiting and diarrhea, which superficially resemble the effects of certain intestinal infections. Accidental poisoning, especially on a large scale, was sometimes confused with infectious disease and other times blamed on witchcraft.
Hereditary diseases are the result of genetic defects passed on by one’s parents. People born crippled were frequently viewed as victims of divine displeasure (usually directed against their parents), and those who exhibited strange behavior, such as epileptics, were often seen as possessed by spirits (evil or good, depending on their society’s outlook). Nonetheless, genetic defects are rare compared to other causes of disease and probably caused little confusion, even though they were not understood until recently.
Cancers tend to occur later in life. They happen because of a build-up of genetic damage over the years in nonreproductive cells. These genetic defects are thus not passed on to the children, but instead are confined to the multiplying cancer cells within a single person. Cancer cells grow out of control and destroy the body to which they belong. Toxic chemicals, both natural and artificial, and ultraviolet radiation from the sun are responsible for many cancers. Other cancers happen because the body’s own genetic machinery makes occasional mistakes. Cancers draw notice only when people live long enough for genetic damage to accumulate. Until recently, cancers were responsible for an insignificant fraction of human deaths. Death by heart disease, stroke, or old age is largely a modern luxury. Historical populations rarely lived long enough or ate well enough for their arteries to clog with fatty deposits.
Infectious diseases from microorganisms have caused most deaths by far throughout recorded human history. In this respect, our own age is peculiar. Thanks to modern technology, we mostly live long enough to worry about heart disease and cancer. But for most societies throughout history, most people met their end from infections caused by microorganisms of some kind. This is still true for some Third World countries. Despite this, scientists have understood the nature of infectious disease only since the late 1800s.
Infectious disease is caused by invisible microorganisms
The cause of infectious disease—and even whether diseas
es were actually contagious and could be passed from person to person—has been hotly disputed over the ages. Only during the late 19th century could science begin to investigate the microorganisms that cause disease. In early times, infectious disease was often seen as punishment from the gods. Later, disease was blamed on such things as night air, marsh air, or other vapors. This attitude is well illustrated by the phrase “You’ll catch your death of cold.” As we now know, “colds” are caused by viruses, not exposure to low temperature. Nonetheless, poor nutrition, poor housing, and exposure to extremes of heat or cold weaken potential victims. Dirt may not literally breed disease as once thought, but lack of hygiene allows germs to survive and spread.
Germs, Genes, & Civilization: How Epidemics Shaped Who We Are Today Page 4