Observations on recent studies showing increased co-occurrence of autoimmune diseases.

The co-occurrence of autoimmune diseases has been epidemiologically studied and has aided in our understanding of autoimmunity. However, as new perspectives develop on the pathogenesis and natural history of autoimmune diseases, a refinement in the methodology for the study of the co-occurrence of disease is warranted in order to maximize the information that one may realize from such studies. This paper presents some recent results of co-occurrence studies and then proposes several refinements in the design of epidemiological studies in light of current understanding of the natural history of autoimmune diseases. It also suggests an historical perspective on the results of past studies as to the type of information that can be inferred from them.