We are their food. Those germs of the past that best converted our bodies into their own propagation are the germs of the present. Those germs of the present that best convert our bodies into their own propagation will be the germs of the future. Why should we care about the prospects of one particular germ over another? Aren't they all just plain bad? The simple answer is no. We can never get rid of them all. Their future is our future. If their future goes one way, we will be relatively healthy; if it goes another, we will be sick or even dead. So the question is, how will they survive? Or rather, how will they evolve?

Surprisingly, neglect of the germ's-eye view of the world is not restricted to the average person; it extends to medicine as a whole for most of its history. Only during the past twenty years have researchers emphasized the importance of looking at a germ's evolutionary

score-card. This scrutiny is suggesting solutions to the most damaging problems of medicine as well as the most irritating. Both categories of problems are important. AIDS, tuberculosis, and malaria are important because they are so damaging; though most of the people reading this book will not suffer from these diseases, they are common enough that our lives are affected indirectly. On the other hand, the common cold is not life-threatening, but it is important because it is such a pervasive nuisance.

Disease from the germ's perspective has been best worked out for the acute infectious diseases. These are the diseases most of us picture when someone mentions infectious

diseases—the common cold, strep throat, pneumonia. They typically arise suddenly, within a week or two after the germ has invaded, and are generally controlled by our immune system within a few weeks.

Typically, acute infectious diseases turn quick profits for short-term gain. The pathogens that cause them are corporate raiders, out to get rich quick rather than maintain the health of their targets. If they depend on their host company's well-being, they may have a fairly benign effect. But if the chance to exploit and move on arises, they take it, and the host company suffers and may even be destroyed. Biological parasites take food rather than money, and they spend the food on reproduction rather than material goods. For either kind of parasite—microbial or human—exploitative propensity depends on whether a relatively healthy host is needed for the leap to the next host. When a sick host suffices, the most damaging parasites can prosper.

The germs cannot consciously plan their moves in the way a corporate raider does. But natural selection molds the pathogens so that they act strategically, almost as if they were making plans. The strategic options can be envisioned as a competition that is played out in two contests. The first contest occurs within the host, where the favored competitors are those that most effectively use the host as food for their own reproduction. The second contest is played out in the transmission of pathogens to new hosts; those pathogens that have been successful at growing within hosts are now in competition to reach the remaining uninfected members of the society. A pathogen that never gets transmitted to a new host is doomed; if it is not destroyed by the immune system, its finite future is guaranteed by the inevitable death of the host in which it resides.

These two contests require different talents. The best competitors are those that do well enough at both events to generate the most progeny—thus dominating the next round of the cycle. Natural selection assesses the strengths and weaknesses of competing pathogens much as judges of a decathlon assess the strengths and weaknesses of competitors in different arenas. With natural selection, however, the "points" are copies of genetic instructions. Pathogens earn these points only by propagating through time. A pathogen that takes so much from a host that it compromises its ability to get to the next host may leave fewer descendants than a less gluttonous pathogen. The more frugal competitor may produce fewer progeny within a host but by keeping the host relatively healthy, it may be better transmitted to the next host if, for example, being terribly sick hinders transmission. If a pathogen relies on some well-placed sneeze in an office or a classroom for transport, a person sick in bed would be a disaster. But the converse could also be true. If a pathogen does not rely on a mobile host, then the more gluttonous competitor may have the higher score in the decathlon of evolution. In other words, because the genetically encoded characteristics of a germ that help it win the competition at one stage of the process might hinder it at another, evolutionary biologists consider the trade-offs that are associated with each characteristic. Growing rapidly inside a person typically involves an evolutionary trade-off: the benefit of generating more progeny within a person is weighed against the reduced chances of contacting a susceptible person if the infected person is too sick to move around. Evolutionary biologists are efficiency experts, always assessing benefits relative to costs.

The basic evolutionary principles underlying such analyses have transformed the modern understanding of infectious disease and promise to transform medicine itself because they reveal a fundamental misconception about disease. Throughout the twentieth century, leading authorities in the health sciences believed that coevolution of pathogens with their hosts would inevitably lead to benign coexistence. They arrived at this mistaken conclusion because they did not consider the trade-offs that were a part of the competition. They focused on the long-term survival of particular parasite species as a whole, rather than the success of particular competitors within the species. The mistake is partly attributable to the catchiness of the phrase "survival of the species." With this phrase jingling around the brain like a pop-song refrain, many medical authorities decided that natural selection somehow directly favored the survival of a species. It does not. Rather, natural selection operates through differences in the rate at which certain genetic instructions are passed on relative to different genetic instructions that occur in other individuals of the same species. The eventual survival of the species may be favored or disfavored as a result, but species extinction, if it eventually occurs, is powerless to influence the course of any competition that occurs prior to the extinction. Natural selection obtains its power from the differences in the survival and reproduction of the competitors within a species, which in turn determine differences in the passing on of the genetic instructions that individuals house. That is where one must look if one wishes to understand why infectious diseases are the way they are and what we can do to control them, because that is where the strategies of pathogens are being shaped.