In modern industrialized societies degenerative diseases such as diabetes, cancer, Alzheimer's etc. have a much higher prevalence today than in the past, and a higher prevalence than in Third World societies. This development requires an explanation.
One idea is that modern society allows individuals who have the diseases to survive and reproduce; therefore, the genes for these diseases are not culled by natural selection, unlike in primitive societies where the hardships of life allow only individuals with a healthy genetic predisposition to survive. In other words this is supposed to be evidence and consequence of genetic deterioration of populations. A proposed solution is eugenics, or genetic engineering to sanitize our genetic makeup.
Another idea is that in modern societies individuals face physical and psychological stresses, e.g., large crowds, pollution, electromagnetic waves, etc. which cause disease, because we are maladapted to the environment we have created for ourselves. The rise in degenerative disease is seen as a consequence of evironmental deterioration. A proposed solution is to clean the environment, and make technologies more "natural".
There can be no doubt that both effects may in fact explain (partly) the rise of degenerative disease. But, both of them fail to take a third factor into account, which I suspect may be more important than both. Even if there was no genetic deterioration or environmental deterioration, the rise of degenerative disease would still be seen in populations of modern societies.
This is a simple consequence of the one reality of life, namely death. The total mortality of the population is always 100%, as we all eventually die. Therefore the prevalence of a particular disease (which brings us closer to death) does not depend exclusively on factors (genetic or environmental) that cause the disease. It also depends on other diseases or lethal factors which might beat them to the punch.
In previous centuries for example, death during childbirth, tuberculosis, death in war, famine etc. were more prevalent. Thus, they took a larger part of the 100% pie. Degenerative diseases were thus correspondingly less important, because irrespective of genes or environments affecting them, we used to die of other causes.
Therefore, we must pause whenever we hear alarming statements about the supposed disease-causing effects of genes or environments: if a concrete causative explanation is established between a genetic variant (e.g., sickle-cell trait) or environmental influence (e.g., smoking) then we must accept this explanation. But, when it is maintained that such genetic or environmental causes must exist due to temporal trends in disease prevalence then we must question the premise that genes or environments have anything to do with these trends at all, since such trends can occur even if genes or environments have nothing to do with the particular disease in question.