It occurs to me, as I write this, how much this framework—germs, cells, risk—still scaffolds the diagnostic art in medicine. Each time I see a patient, I realize, I am probing the cause of his or her disease through three elemental questions. Is it an exogenous agent, such as a bacterium or virus? Is there an endogenous disturbance of cellular physiology? Is it the consequence of a particular risk, be it exposure to some pathogen, a family history, or an environmental toxin?
