Plague Time was written to provide a broad view of the threats posed by infectious agents, to distinguish these agents according to how much of a threat they pose, and to suggest ways to reduce these threats. It has received some praise and some criticism. Assessments of the criticism help reveal why we are where we are in the understanding of human disease. One reviewer, for example, repeated the old adage that correlation is not causation—an association between a particular infectious agent and a particular chronic disease does not mean that the agent causes the disease. True, but this criticism misses a major theme of the article, which uses this adage as a starting point and asks, "How do we structure our guidelines for understanding disease causation when we cannot obtain anything better than correlation?" Indeed, virtually all of the recent acceptance of infectious causation of human diseases, from cervical cancer to peptic ulcers to AIDS, has been based largely or entirely on correlative evidence. All indications are that this situation will not reverse in the future. And it would be unwise to insist on experimental manipulations of a candidate cause before we accept a causal argument, particularly when such manipulations are not feasible. Consider astronomy; where would we be today if astronomers were unwilling to draw conclusions about causation from correlative evidence? If astronomers could experimentally alter the speed or composition of stars and galaxies their conclusioins could be more rigorously tested, but they have been able to make pretty good progress without such manipulations. The health sciences have a greater potential for experimental manipulation of proposed causes than astronomy, but this greater potential does not mean that experimental manipulation will resolve the validity of all causal hypotheses. Science, whether health science or astronomy, needs to draw conclusions on the best evidence and reasoning that are available. Hypotheses about disease causation need to be accepted or rejected on correlative evidence when experimental manipulation is not feasible. One of the messages of Plague Time is that we have now entered a period of medical history when the best standard of evidence for disease causation must rely, as in astronomy, on correlational evidence.
The same reviewer also criticized Plague Time for being written twenty years too early. Now that is a criticism I can embrace. Plague Time was written not to be a bland recounting of the accepted ideas about disease causation, but rather to provide a glimpse of the future promise of medicine. It is the promise of understanding not only the true scope of infectious causation but also how we can pacify the infectious agents, which have killed the vast majority of humans in rich and poor countries alike throughout history and still appear to be doing so.
Another reviewer writing in the American Scientist argued that the scope of the book was too broad. This criticism illustrates my argument that medicine has been hampered by the incessant forces favoring specialization, pressuring researchers to focus on particular trees—often the molecules within the cells of the leaves of the trees—instead of the forest.
The need for breadth is one of my reasons for writing Plague Time. To emphasize this point its subtitle has been changed to "The New Germ Theory of Disease" (from "How Stealth Infections Cause Cancers, Heart Disease, and Other Deadly Ailments). The new germ theory does not discard and replace the old germ theory but broadens it to incorporate the full spectrum of insights that science can now bring to bear on disease causation and control. The new theory emphasizes evolution, making evolution's role in the medical study of life consistent with its role as the central organizing principle for the scientific study of it. In so doing, this new germ theory provides fresh insights about the causes of disease because it distinguishes causal explanations that are inconsistent with evolutionary principles and therefore are not feasible from those that are. It also presents innovative solutions to the most intractable diseases by suggesting ways in which evolutionary changes in pathogens can be part of the strategy for disease control rather than an impediment, as has been the case with the evolution of antibiotic resistance. The new germ theory looks forward to consider new strategies, but it also looks to the past to discover clues that the health sciences may have passed by in its rush to embrace bad genes, bad chemicals, and bad development as the sources of all poorly understood diseases, as well as the mechanistic branches of biology and technology as the sources of all solutions.