In contrast, the percentage of elderly increased fairly constantly over the century. Persons ages 65 and older made up just 4 percent of the U.S. population in 1900 and nearly 13 percent in 1997. The mortality transition in more developed countries involved a shift in the major causes of death from such communicable diseases as measles, influenza, and smallpox, to chronic and degenerative diseases such as heart disease, cancer, and emphysema. This fundamental change reflected a broader change in health status and health threats that is described as the epidemiologic transition. 17 Scholars define the transition in various ways, but most identify several stages of transition as mortality rates fall.
Death rates are high, but spike even higher during health crises.
This stage is followed by the age of receding pandemics. Death rates fall rapidly during this stage as infectious diseases are controlled. Next is the age of degenerative and man-made diseases, during which the death rates typically fall slowly to very low levels. Countries around the world occupy different stages in this transition. One reason that noncommunicable diseases such as cancer and heart disease cause a smaller percentage of deaths in areas in the early stages of transition is that a smaller proportion of the population is in the older ages, where the risk of death from these degenerative diseases is greatest.