Case-case comparisons to study causation of common infectious diseases.

BACKGROUND Analytical studies of reportable infectious diseases often use the small minority of cases detected through surveillance systems. For many diseases, notification of cases represents a non-random selection process. Apparent differences in exposure histories may be due to biases involved in the surveillance system selection of cases compared to randomly selected controls. In addition, differential recall between cases and controls may occur. One way to avoid these problems is to compare cases with another group of cases with a different disorder selected by a similar surveillance system, although this can introduce new biases. METHODS In infectious diseases cases with the same disease can be divided into aetiologically meaningful subgroups by subtyping the pathogen. Exposure history can then be compared between these subgroups. RESULTS Several biases are removed. The control group composed of other cases does not represent the exposure history of the study base but differs from it in a predictable and useful way. People considered as controls will have a higher incidence of general predisposing factors than the general population. Analysis is limited to factors associated with exposure to the infecting agent. CONCLUSIONS Case-case comparison is a development of case-control methodology made possible by laboratory typing techniques. These comparisons allow a more restricted but more refined analysis of the association of some exposures with infection. Determination of how exposure to the infectious agent occurred is more efficient and unbiased than in standard case-control studies but general factors determining whether disease occurs after an infectious exposure can not be studied.