the book was a valuable contribution in its time. It offered no erudite mathematical models but it spoke plainly on the subject of what disease scientists do, and what they should do. What they should do, by his lights, was to think about infectious pathogens in ecological and evolutionary terms as well as medical ones.
Parrot fever was one of his exemplary cases. It had the attractions of an Australian connection (for him, a local bug) plus global reach, and it illustrated a favorite point. “Like many other infectious diseases, psittacosis was first recognized as a serious epidemic disease of human beings, but as its nature became gradually understood, it grew clear that the epidemic phase was only an accidental and relatively unusual happening.” The bacterium had its own life to lead, that is, of which infecting humans was just one part—and arguably a digression.
Burnet retold the tale of the California-bred parakeets, the wild Australian cockatoos, the infection of working-class Melbourne bird fanciers by animals sold out of Mr. X’s dismal backyard shed. Psittacosis, Burnet noted, is not normally very infectious. It exists endemically among wild bird populations, causing little trouble. One could reasonably suppose that “those cockatoos, left to a natural life in the wild, would never have shown any symptoms.” But the bird catcher, and then Mr. X as middleman, had disrupted their natural life. “In captivity, crowded, filthy and without exercise or sunlight, a flare-up of any latent infection was only to be expected.” The stressful conditions had allowed Chlamydophila psittaci (as Rickettsia psittaci later became known, after another of those taxonomic revisions) to replicate and erupt.
This case and similar ones, Burnet wrote, embodied a general truth about infectious disease. “It is a conflict between man and his parasites which, in a constant environment, would tend to result in a virtual equilibrium, a climax state, in which both species would survive indefinitely. Man, however, lives in an environment constantly being changed by his own activities, and few of his diseases have attained such an equilibrium.” Burnet was right on the big ideas, including that one: environmental disruption by humans as a releaser of epidemics. Still, he couldn’t foresee the particulars of what would come. Publishing in 1940, he focused on several infectious diseases in addition to psittacosis: diphtheria, influenza, tuberculosis, plague, cholera, malaria, yellow fever. These were the old, familiar, infamous scourges, fairly easy to recognize though not well enough understood. Our modern age of emerging viruses was just beyond the reach of his headlights.
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Burnet didn’t mention Lyme disease but, because it shares one important characteristic with Q fever and psittacosis, I will. The most fundamental thing about this newly emergent or re-emergent infection is that it’s not caused by a virus. The Lyme agent, like Coxiella burnetii and Chlamydophila psittaci, is an anomalous, crafty bacterium.
Lyme disease is hotly controversial, though, in a way that Q fever and psittacosis are not. Segments of the scientific and medical communities, plus victims or supposed victims, can’t even agree (especially they can’t agree) on who has the disease and who doesn’t. Roughly thirty thousand cases of Lyme disease were reported in the United States during a recent year, and more than twenty thousand per year as a ten-year average. You probably know someone who has had it; you may well have had it yourself. By any standard, it’s the most commonly reported vector-borne disease in the United States. But do those thirty thousand cases in one year represent the true total of affected Americans or only a small fraction of the real number of cases, most of which go undiagnosed? Is there such a thing as “chronic Lyme disease,” which eludes detection by conventional diagnostics, persists despite prescribed treatment with antibiotics, and causes gruesome suffering among people who can’t persuade their doctors or their insurance companies that they are genuinely infected? Does Borrelia burgdorferi hide in the body and somehow later recrudesce?
Disagreements on such points have stretched all the way from the examining room to the courtroom, making Lyme not just the most common infection of its kind but also the most confusingly politicized. For instance, in 2006 the Infectious Diseases Society of America suggested in its guidelines for treatment that “chronic Lyme disease” is an illusion. More precisely, the IDSA wrote: “No convincing biologic evidence exists for symptomatic chronic B. burgdorferi infection in patients after recommended treatment regimens for Lyme disease.” The recommended treatment regimens, involving two to four weeks on an antibiotic (such as doxycycline or amoxicillin), should cure