From the book
From the book
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, spreads 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.