This essay examines the efforts of travelers such as John Fryer, John Ovington, and Alexander Hamilton, and physicians, including John Arbuthnot and James Lind, to explain the seasonal pathologies of the monsoonal tropics—the diseases that swept through late-seventeenth-century Bombay during the rainy season and again during the intense heat of the tropical summer. The high rates of mortality for merchants and sailors forced the British to reassess their fundamental assumptions about the relationships among climate, ecology, and human health. Lacking an understanding of microbial biology, the efforts of these writers to locate the sources of diseases assume a Hippocratic, cause-and-effect relationship between sick bodies and a pathogenic air. Yet the deadly illnesses they encountered in Bombay reveal the extent to which the tropics signify differently in the East and West Indies. In contrast to the Americas where cultivating land improves the climate, the diseases of the monsoonal tropics register the vulnerability of the British and, before 1757, the tenuousness of their position as interlopers at the margins of the Asian trade. Indian civilization had acclimated itself to the rhythms of trade and commerce dictated by the monsoons and thereby seems to lie outside of, and threaten, European moral and climatological economies.