Figure 1.

Chronologic aspects of comorbidity.

Each block represents the duration of a different comorbid disease. Two comorbid diseases can either be present at the same point in time (vertical arrow), or occur within a given time period without being simultaneously present at any given point in that period (horizontal arrow) (a). Irrespective of the selected time span, the sequence in which the diseases appear is of particular interest in the study of etiological association (b).