1. Aging of the general population: because of improved medical care, the impact of mortality from diseases reduces. There may be greater uncertainty about the precise cause of death.

2. Changes in diagnostic criteria/reporting: criteria changes over time due to increased knowledge about a disease, some categories of diseases used earlier in time may be omitted altogether.

3. Changes in disease treatment and cure: disappearing disorders, residual disorders, persisting disorders, new epidemics.

4. Changes in population dynamics (migration): a) births, b) deaths, c) migration. When these factors don't contribute to net increases or decreases, the population is in equilibrium.

Review: Epidemiology, Chp. 2 - Practical Applications of Epidemiology