The phrase "the demography of Alzheimer's disease" may be the best litmus test to distinguish demographers from epidemiologists. A typical reaction to this phrase from a demographer is "why Alzheimer's disease?" A typical reaction from an epidemiologist is "why demography?" It is always tricky to describe the exact boundaries that separate two similar disciplines and this is especially true of the difference between demographic and epidemiologic approaches to mortality and health. What made Preston's work on smoking and mortality demography rather than epidemiology? What differentiates a demographer's or a sociologist's work on factors affecting residence in nursing homes from an epidemiologist's work on the same topic?