Virus X: Tracking the New Killer Plagues Out of the Present and Into the Future, by Frank Ryan, is an intelligent and writerly book that traces the phenomenon of emerging viruses and proposes an evolutionary explanation. A sort of clinical companion to Christopher Wills's Yellow Fever, Black Goddess, Ryan uses evolutionary biology to help explain some of the sudden outbreaks of extremely lethal organisms that we are seeing today. Virus X overlaps in themes and cases with C.J. Peters's Virus Hunter, as well, but it takes a somewhat more distant view, providing larger contexts for the themes of dwindling budgets and rising human vulnerability to disease.

Virus X concentrates on three viruses: the Sin Nombre hantavirus, which broke out in the Four Corners region in 1993; Ebola in its many forms; and human immunodeficiency virus (HIV). Ryan's descriptions of the events surrounding the outbreaks of these viruses is gracious and articulate. He expresses the breadth of expertise that is required to understand and contain such diseases, and he manages to convey the painstaking quality of the work in a book that is paced like a thriller.

For all Virus X's perspective on the laboratory aspect of epidemiology, it has a decidedly clinical focus, which Ryan describes with appropriate scope -- clinical medicine involves the laboratory, but it also encompasses the experience of the physician, the local availability of materials, the willingness or ability of patients to comply with treatment, the funding environment, the political landscape, and the media. While the growing resistance to antibiotics and the tabloid fare of "flesh-eating bugs" help to show this, nowhere is it more apparent than in the case of HIV, whose very structure -- HIV-2 is fundamentally the same as simian immunodeficiency virus (SIV) -- brought political correctness into play as well.

Still, the basis of effective clinical treatment is effective treatment, and the action of Virus X is never very far from the causes of the diseases it discusses. Sin Nombre hantavirus, at once the most recent and least mysterious of the viruses discussed at length in this book, provides the clearest example of both the multidisciplinary approach needed to contain epidemic disease and the argument that Ryan offers to explain the apparent explosion of such outbreaks: aggressive symbiosis. Viruses and bacteria are under tremendous evolutionary pressures, and their numbers, mutation rates, and ability to recombine genetic information allow them to develop stunning arrays of strategies very rapidly. As Ryan shows, this has implications beyond resistance to antibiotics.

In Yellow Fever, Black Goddess, Christopher Wills describes in detail the vulnerability of monocultures to devastating disease, and he describes scientific investigation of the protective role of variety in populations -- from the highly variegated landscapes of natural-state plant environments to the variety of the immune protection factors in humans. Ryan offers the flip side of this scenario: symbiotic relationships between organisms where one organism actually behaves aggressively in a way that protects its symbiont. Ryan notes that some organisms live in cooperative arrangements in which one may obtain shelter and the other protection -- such as plants that harbor insects that bite herbivores. While varied populations may be less vulnerable, aggressively symbiotic relationships can serve not only to "protect" an organism (by delineating boundaries) but, more than that, to endanger predators. It's the overtly biological support for the "we're trespassing on the rainforest" argument, and it is nicely supported scientifically as well as being aesthetically appealing.

Virus X is a thought-provoking book that suggests that the dreaded doomsday scenario is, for all practical purposes, already here: in a highly mobile population concentrated in large cities, only a few "trespasses" may be sufficient to disseminate an aggressively symbiotic organism. Like HIV. And clinically, it's not about "slate-wipers", it's about human suffering and the complacency that allows it to continue. The exigencies of the science involved are nicely interwoven with the larger clinical picture, particularly the reduction in funding available for epidemic surveillance -- and for training of the next generation of epidemiologists.