This article retraces the historical origins and contemporary resonances of Rudolf Virchow's famous statement "Medicine is a social science, and politics nothing but medicine at a larger scale". Virchow was convinced that social inequality was a root cause of ill-health, and that medicine therefore had to be a social science. Because of their intimate knowledge of the problems of society, doctors, according to Virchow, also were better statesmen. Although Virchow's analogies between biology and sociology are out of date, some of his core ideas still resonate in public health. This applies particularly to the notion that whole populations can be sick, and that political action may be needed to cure them. Aggregate population health may well be different from the sum (or average) of the health statuses of all individual members: populations sometimes operate as malfunctioning systems, and positive feedback loops will let population health diverge from the aggregate of individual health statuses. There is considerable controversy among epidemiologists and public health professionals about how far one should go in influencing political processes. A "ladder of political activism" is proposed to help clarify this issue, and examples of recent public health successes are given which show that some political action has often been required before effective public health policies and interventions could be implemented.