Rats have long been one of the world’s most ubiquitous—and infamous—forms of urban wildlife, synonymous with pestilence and squalor. They’ve attracted only sporadic attention from scientists, however, and much about the secretive city rat—chiefly the Norway rat, Rattus norvegicus, remains a mystery. But as the world’s urban population surges, and more people crowd into rat-plagued slums, the rodents are getting renewed attention from researchers and public health experts. Over the past decade, scientists in a number of cities have launched efforts to better understand rat behavior and evolution, and the role they play in spreading disease. One of the most intensive and longest-running investigations is occurring in Pau da Lima, a crowded favela on the edge of Salvador, Brazil’s third largest city. For the last 2 decades, researchers have been working to understand the forces driving leptospirosis, a bacterial disease spread by rats, which kills some 60,000 people annually worldwide. They are searching for the best ways to curb a disease that experts warn is an underappreciated threat in the burgeoning slums of a more urban world.