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

by David P. Clark

  An intriguing aspect of historical beliefs about infectious disease is that the common folk were proved to be right in the long run, and the educated were mostly wrong. The priesthood pushed the idea that disease came from the gods. People were told to stop sinning and to pray for forgiveness, not waste time attempting to understand disease. Rationalist intellectuals put forward a range of theories based on factors such as diet, personality, climate, dirt, decay, and offensive odors of various sorts. Until the last century or two, most intellectuals rejected the idea that disease was contagious.

  However, the behavior of the population-at-large suggests that ordinary people were aware that disease was often contagious. Avoiding contact with those infected by typhoid, plague, smallpox, and malaria was a sensible precaution. During the 1600s, the wealthier inhabitants of London kept an eye on the weekly “Bills of Mortality,” much as we tune in to the weather report nowadays. These “bills” were lists of recent deaths and their causes. When the number of cases of something especially nasty, like plague or smallpox, rose higher than normal, the wealthy fled London for their country estates and left the poor to take their chances.

  Why did the scientific establishment take so long to realize that diseases are transmitted from one victim to another? I believe two factors are at work. First, many diseases are not directly contagious. Thus, although malaria is spread from person to person, it is carried by mosquitoes, and a person cannot catch it through direct contact with a human sufferer. Bubonic plague is even more confusing. It can be spread from person to person, but it is usually transmitted by fleas. From a practical viewpoint, avoiding those infected is still a good strategy—you would be less likely to be bitten by the same flea or mosquito. From an intellectual viewpoint, the observed lack of direct transmission favored the various environmental theories. Second, the technology to actually see microorganisms is of relatively recent origin. Speculation about tiny invisible germs goes back to the Roman author Varro (116–26 B.C.), but demonstrating their existence requires more than mere words: It requires a microscope.

  How infectious disease spreads

  Different contagious diseases spread in different ways. We can subdivide these into three major mechanisms. Some diseases spread by direct person-to-person contact. Others spread indirectly via inanimate objects. Yet a third strategy is for insects or other intermediaries to carry the infectious agent. The way an infection spreads greatly affects whether it becomes milder over the ages, stays much the same, or gets more virulent.

  Certain diseases require prolonged contact of an intimate nature to move from one person to another. These diseases are relatively hard to catch and can often be avoided by changing personal behavior. The sexually transmitted diseases (STDs) such as syphilis, AIDS, and gonorrhea illustrate this scenario. Strictly speaking, the transfer of body fluids is involved here. This is important from a practical viewpoint, because such infections can also be spread by improperly sterilized hypodermic needles. This occurs both among the intravenous drug users of the industrial world and in the clinics of Third World nations that lack money for disposable syringes.

  Other diseases are spread by direct personal contact, but with less intimacy than for STDs. Many venereal diseases probably evolved from ancestors who infested the skin and body surface in a more general way. For example, the chlamydia that infect the genitalia are closely related to those causing the eye infection trachoma. The specialized sexual versions likely arose in historical times only as human populations became denser (see Chapter 7, “Venereal Disease and Sexual Behavior”).

  Some germs are transmitted by bodily contact or via nonliving objects such as doorknobs, paper money, clothes, and bed linen. Highly contagious virus diseases such as colds, influenza, measles, and smallpox are typical of this group, although most of these can also be transferred through the air. Many infections are transmitted from person to person through the air by coughing or sneezing. This is known as droplet transmission, because the germs are carried in microscopic droplets of saliva, phlegm, or mucus. Many of these germs fail to survive if they dry out completely. Tuberculosis, influenza, and colds are familiar examples. As the nursery rhyme says:

  I sneezed a sneeze into the air,

  It came to ground I know not where.

  But hard and cold were the looks of those,

  In whose vicinity I snoze.

  Infectious agents can also be taken in with food or drink. Poor hygiene may result in food or drinking water being contaminated with human or animal waste. Typically, such infections affect the gastrointestinal tract and include the many types of protozoa, bacteria, and viruses that number diarrhea among their symptoms. The purpose of diarrhea, from the germ’s viewpoint, is to provide an exit mechanism from the body and to recontaminate the water supply. Examples of waterborne diseases include Cryptosporidium (a protozoan), cholera (a bacterium), and polio (a virus). Infections caught from food are often referred to as “food poisoning,” despite resulting from bacteria or viruses instead of poisonous chemicals.

  Diseases are often carried by insects such as mosquitoes and flies or by animals such as rats and mice. These are referred to as vectors. Sometimes multiple vectors are involved, such as in the spread of the Black Death by fleas carried by rats or typhus fever by ticks carried by rodents. Controlling vectors usually limits the spread of a disease far more effectively than treating infected humans. Insects and their relatives, the ticks and mites, are the most common vectors. However, other animals may act as vectors, as in the spread of rabies by bats and squirrels, or of West Nile virus by migrating birds. Plague and typhus normally rely on fleas and ticks to distribute them, although, under some circumstances, they can spread from person to person. Other diseases are obliged to spend part of their life cycles in a second host. Thus, malaria must pass from human to mosquito and back again to complete its developmental cycle.

  Many diseases become milder with time

  Let’s consider the spread of a virulent virus like Ebola from the viewpoint of the virus. After infection, the victim will most likely die in a few days. Before the first victim dies, the virus must find another victim to infect. Clearly, the longer the first victim moves around, the greater the chances are of the virus making contact with someone else. If the virus incapacitates the first victim too quickly, it will undermine its own transmission. Consider, too, the spread of the virus from village to village. As long as the virus stays in the same village, where plenty of potential victims live close together, it can get away with killing fast. But what happens when the village has been wiped out? The virus must now find another population center. This requires an infected person who is still fit enough to travel. Over the long term, movement between population centers may matter more than how a disease spreads locally within a group of people.

  Now consider two slightly different Ebolaviruses. One kills in a day or two. The second takes a whole week. Virus 1 may wipe out a whole village, but it will find it very difficult to transfer itself to the next village. Even if a dying victim staggers within sight of the next village, its people will probably not allow him in. During plague epidemics in medieval Europe, many villages and small towns stationed archers to intercept travelers. Anyone showing symptoms of plague was warned away and shot if they ventured too close. While lacking in sensitivity, such quarantine measures were effective, and many small villages escaped entirely from epidemics that decimated nearby towns.

  By comparison, a less virulent Ebolavirus will spread much more effectively. Infected refugees fleeing an infected village may reach another center of population before symptoms appear. Thus, if we have a mixture of viruses, the milder forms will spread more effectively and, over time, will predominate. Many diseases appear to have done just this and have evolved to become milder. Examples include gonorrhea and syphilis (caused by bacteria), and measles, mumps, and influenza (caused by viruses). What unites these diseases is that all are transmitted directly from person to person.

  Ebo
lavirus infects humans now and then after emerging from some animal host, probably bats. It wipes out a few people in close contact, and then the mini epidemic burns itself out. Much the same is true of Lassa fever and other highly virulent diseases that burst out of the jungle every so often. Although they give the press the opportunity to spur apocalyptic hand-wringing, they are unlikely to spread far without getting milder.

  Crowding and virulence

  Earlier thinking held that, given time, all diseases would adapt, to become no worse than measles and mumps. Virulent diseases were newcomers, not yet adapted to a state of biological détente with their human hosts. This viewpoint sees man and his infections in a perpetual cold war, with casualties due only to occasional misunderstandings. This wishful thinking has obvious marketing appeal and still frequently appears in books and articles that popularize biology.

  This scenario ignores the ugly side of both evolution and human history. The inhabitants of our history books did not merely suffer from childhood diseases while their mothers read them stories about rabbits and mice dressed in human clothes. Until our own privileged age, most people died of infectious disease, much of which small rodents spread. The purpose of evolution is not to make life better for humans, nor even to produce a balanced ecosystem. Indeed, the very idea that evolution has some underlying moral purpose is basically religious. Evolution is simply a mechanism by which different living things compete using various genetic strategies. Those that propagate their own kind more effectively increase in numbers, and the less efficient go extinct. Mother Nature has no maternal instincts.

  No absolute reason exists for why a disease should not remain virulent, nor why it should not get more virulent. Some do. Indeed, the same disease may fluctuate in virulence as conditions change. The critical issue is which factors promote decreased virulence and which promote increased virulence. The two main factors are overcrowding and transmission mode. Consider again two variants of the same disease, one mild and the other virulent. If humans are closely crowded, the virulent version has the advantage: There is no need for the patient to linger for several days to pass on the germs. As long as plenty of new victims are available nearby, the best strategy is for the disease to grow as fast as possible inside the original victims, generating more germs to infect more people. The slower, milder version of the disease will be left behind. Diseases tend to grow in virulence when their hosts are plentiful and crowded closely together. Conversely, diseases evolve with lesser virulence when their hosts are few and far between.

  A highly virulent epidemic may wipe out a substantial portion of the human population. This decreases crowding, which, in turn, selects for a decrease in virulence. Ultimately, you might think, a balance will be struck and both the population density of the host and the virulence of the infectious agent will settle down to a gentlemanly compromise. This is the microbiological version of the famous “balance of nature” myth. But instead of reaching a state of stable equilibrium, periods of population growth generally alternate with devastating epidemics. Chinese records illustrate this effect. Between 37 A.D. and 1718 A.D., 234 outbreaks were severe enough to count as plagues—that’s one every seven years. Although not every epidemic covered all of China, the frequency is impressive.

  Bubonic plague provides a nice example of a disease whose virulence oscillated. Beginning in the mid-1300s, repeated epidemics of bubonic plague swept across Europe until the 1600s (later in some places). When plague first reached a town or city, the first few cases were usually mild and the victims recovered. Once within the crowded confines of a town, the plague became more virulent, often switching to its pneumonic form, which is spread through the air by coughing. Anyone who caught pneumonic plague could be dead within a day. From the germ’s viewpoint, this is no problem, provided humans coughed germs over and infected another victim within this time. In a crowded medieval city, this was normally the case. Toward the end of an outbreak, most of the population either was dead or had recovered and become immune. Hence, the plague became milder again as the number of available victims became fewer and farther between. The mild forms then spread to the next city, and the cycle repeated. After a couple generations, the population recovered to where it could provide a sufficient supply of fresh victims, and the plague might revisit the original city.

  Note the time scale. Microorganisms evolve so fast that they can change their minds—or, rather, their genes—during the course of an epidemic lasting less than a year. As illustrated by the Black Death in a single city, mutants with increased virulence may appear and spread in only a few weeks, and the reverse occurs toward the end of the outbreak. Thus, the virulence of a disease such as plague neither decreases nor increases; it oscillates. A major problem for the historian is that if a disease can change significantly in a year, how did it behave a hundred years ago? A thousand? Ten thousand?

  Today the human population is exploding. In many Third World countries, this is exacerbated by poor hygiene. Consequently, we can expect diseases that are efficiently transmitted from person to person to become more virulent. In addition, more people need more food. The tendency is to plant larger areas with the same crop, to improve efficiency. However, such crowding makes crops more susceptible to epidemics, just as with humans. The best-known crop disaster was the Irish potato famine, which resulted from over-reliance on a single crop. When a virulent strain of blight fungus wiped out the potatoes, the Irish had little left to eat. Infectious disease then followed in the footsteps of malnutrition. Starvation itself killed relatively few—most victims died of cholera, dysentery, or typhus fever. Thus, crop failures and malnutrition amplify the effects of infectious disease.

  Vectors and virulence

  Virulence may increase when a vector carries a disease. If a germ hitches a ride from one victim to another via mosquito, it matters little that the first victim is too sick to move. Indeed, this may even work to the germ’s advantage. Mosquitoes will be able to land and suck blood without the victim swatting them. Diseases that are carried from person to person by some other agency have little motivation to evolve mildness toward humans. Rather, they must avoid disabling their carriers. What happens to the human victims is less important. Malaria, sleeping sickness, typhus fever, yellow fever, and many other diseases are spread by insects, ticks, or lice. These diseases are dangerous and show few signs of getting milder. Indeed, the more virulent form of malaria, Plasmodium falciparum, is today spreading throughout the tropics and subtropics from its original focus in Africa.

  The best way to control these diseases is to kill the vectors, thus interrupting transmission. Spraying insecticides such as DDT greatly reduced the incidence of malaria in many areas. Sadly, malaria is making a comeback in many parts of the Third World, due partly to insecticide-resistant mosquitoes and partly to complacency and political disintegration. Irrigation projects such as dams, reservoirs, and irrigation canals often work well in temperate climates. However, in tropical regions, they may backfire. They create large bodies of stationary water that are ideal breeding grounds for the mosquitoes that carry malaria, yellow fever, and other diseases. The slowly moving water of canals also provides a suitable habitat for water snails that carry the parasitic worms causing schistosomiasis (bilharzia). An example was the spread of schistosomiasis during the Senegal River Basin development in West Africa.

  Waterborne diseases use the water itself as a vector. Such diseases can also increase in virulence. The disease relies on contaminated water instead of an insect to carry it from person to person. But the principle is much the same: The disease does not rely on human victims for dispersal. Contaminated water supply normally spreads dysentery, cholera, and many other infections that cause diarrhea. Rivers can carry germs in untreated sewage downstream and infect towns and villages hundreds of miles away.

  Reservoirs and carriers of disease

  A disease reservoir is a source of infection outside the human species. Reservoirs are usually animals in whom the infect
ion is mild or even causes no disease. For example, bats are a reservoir for rabies and probably also for Ebolavirus.

  A carrier is a human who is infected but does not become ill. Although carriers show no symptoms, they may transmit the disease to others. Even if all the susceptible human victims are dead or incapacitated due to a virulent infection, a few carriers may keep the infectious agent in circulation. Carriers may travel from one town to another, or they may stay where they are and keep the disease alive to emerge at some future time. Clearly, a disease that can rely on symptomless carriers or an animal reservoir is under less pressure to become milder.

  Many diarrheal diseases cause symptoms in only a fraction of their human hosts. The proportion of symptomless carriers varies immensely. It may be more than half, as in Cryptosporidium or amebic dysentery, or very rare (about 2%–3%), as in typhoid. In most cases, the germs simply live in the intestines without causing disease. Intestinal diseases in which a large fraction of the population shows no symptoms are, by their nature, relatively mild, at least in most adults. The casualties from such diseases are mostly infants in poor countries. Malnutrition and lack of medical care make infantile diarrhea a major killer under such conditions.

  A few special cases are known of germs that have adapted specifically to inhabit some tissue other than the intestines. In such cases, the disease may remain much more virulent. In typhoid carriers, the bacteria inhabit the gall bladder, emerging now and then into the intestine. From there, they can reemerge into human society. Salmonella typhi, the agent of typhoid fever, is one of the most virulent infections spread by the contamination of food or water with human waste. It is also a specifically human disease, unlike many other varieties of Salmonella, which are shared with assorted animals. These less dangerous relatives have no special hiding place and must therefore refrain from killing their multiple hosts to stay in circulation.