Americans come together to fight diseases. For over 100 years, they have asked their neighbors to contribute to disease campaigns and supported health policies that target one disease at a time. Common Enemies asks why disease campaigns are the battles Americans can agree to fight, why some diseases attract more attention than others, and how fighting one disease at a time changes how Americans distribute charitable dollars, prioritize policies, and promote health. Drawing on the first comprehensive data on thousands of organizations targeting hundreds of diseases over decades, the author shows that disease campaigns proliferate due to the perception of health as a universal goal, the appeal of narrowly targeted campaigns, and the strategic avoidance of controversy. They funnel vast sums of money and attention to a few favored diseases, and they prioritize awareness campaigns and medical research over preventing disease and ensuring access to healthcare. It’s easy to imagine more efficient ways to promote collective well-being. Yet the same forces that limit the potential of individual disease campaigns to improve health also stimulate the vast outpouring of money and attention. Rather than displacing attention to other problems, disease campaigns build up the capacity to address them.