Chapter 4 exposes and explores the prevalence and movement of knowledge of the bark’s effectiveness in ‘fevers’ and other ailments occasioned by ‘insalubrious’, ‘febrile’ environs. Bark knowledge, the chapter contends, spread to various Atlantic localities not only in the form of imaginative stories or culinary practices, as the previous chapters have shown, but also in that of diagnostics, of expertise in indications for the bark and of a topographic literacy of sorts that associated even widely different environments with the same, familiar kind of ‘febrile’ threat. Men and women from all ranks across the Atlantic World and beyond, who inhabited or moved temporarily into ‘insalubrious’ environs, shared an understanding that their ability to preserve or restore bodily well-being was contingent on a litany of precautions and cares. Cinchona bark, this chapter contends, had become a fundamental element of that register by the late 1700s and early 1800s.