A few hundred years ago, a child with diabetes would have been lucky to survive to adult life. Thanks to insulin injections, diabetics now have nearly as much chance as the rest of us to leave their genes to the next generation. The same is true for scores of other diseases. Bad genes, or bad combinations of genes, didn't make it through to the next generation. Now we see most of our offspring provide us with grandchildren, whatever their genetic inheritance.
Natural selection needs the grim reaper.
Without his cruel separation of the fit from the weak, we will grow weak. We are healthier and will live longer than our parents, but our genes are not improving. Modern medicine, and improved living conditions, rescue us from our imperfect genetic inheritance. Each government promises to spend more. Health advisers may pin their hopes on lifestyle changes to reduce the burden of disease, but most of the risk for cancer and heart disease is in our genes.