Why did the lands east of the Urals–long associated by Russians with the hard labor of convicts, fearsome nomads, and an unbearable climate–attract so many Russian peasant settlers at the end of the nineteenth and early twentieth centuries? Historians usually cite a combination of push‐pull factors. They represent migration as a consequence of stagnation in the agrarian economy (push) and state policies promoting migration (pull), rather than interpreting migration as a dynamic element that contributed to these factors. These explanations therefore suffer from being tautological. They are at best incomplete and at worst distort the culture and agency of the migrants themselves. In seeking to comprehend the motivations of the millions of peasants who migrated to Siberia, we confront, much like contemporary observers, an epistemological question: how to know for sure what millions of mostly illiterate people were thinking when, after all, their subaltern position encouraged them to obfuscate, prevaricate, and produce “hidden transcripts?” In this article I argue that “settlement fever” (pereselencheskaia goriachka), a metaphor employed by A. A. Kaufman, Imperial Russia’s leading expert on peasant migration to Siberia, best expresses the diffusion of knowledge about and enthusiasm for resettlement to that part of the empire. In supporting this argument, I draw on the research and insights of Jose Moya concerning analogous “fevers” that impelled more or less simultaneous trans‐Atlantic migrations. I then analyze the Siberian settlement fever’s epistolary and other microbial elements, and conclude by constructing a fever chart for the early Soviet years. This endorsement of Kaufman’s fever metaphor is meant to emphasize an essential element of the migratory process, namely, its communicability. Moya relied on diffusion theory to understand how, like germs, information spread “microsocially,” encompassing ever‐widening “sociogeographic spaces.” He discovered that such factors as proximity to ports and socio‐economic condition paled in significance compared to kinship, friendship, and other microsocial networks. This, I suggest, is how to explain the predominance of “irregular” (samovol’noe) resettlement: peasants went according to their own timetables and scouts. The state adjusted to them, rather than the other way around. Rumor and letters from relatives already settled also played a role. Migration fever abated during World War I and the revolution when it was overtaken by other fevers–self‐demobilization, seizures of landlords’ property, and push‐back from rebellious Kazakhs. It returned though in the 1920s, presumably spreading by the same means as before.