This is a very valid point that is extremely unpopular. Cancer, for example, is a genetic problem. Before billions and billions was pumped into research and treatment then those prone to cancer would die, and the gene would die with them. Now that the survival rate is so high the gene is more likely to be passed on to future generations. The surge in the number of cancer patients is proof of this, even taking into account our environment being more carcinogenic. Consistantly high BMI should lead to a barring from fertility treatment. Your inability to reproduce is natures way of saying thanks but no thanks.