This is exactly true. Many genetic diseases that kill people now, were our genes way of keeping us alive long enough to procreate. Diabetes is actually an example of this. Higher sugar in the blood makes it harder for blood to form crystals when exposed to extreme temperatures. Which meant that people with higher blood glucose levels could survive better In cold climates, and made northern migration into europe possible. "Survival of the Sickest" is a great book on this exact topic.
Human DNA doesn't attempt to "evolve". There is nothing coded in our genes that tells us to change. We pass on half of our DNA to our offspring, and they get the other half from the other parent. This causes the slight difference, but genetic disorders occur when a mutation, or mistake, is made in that process of attempting to perfectly construct the DNA molecule. Genetic disorders are failed attempts of DNA to replicate, not to evolve. They are mutations that just happen to stick around and are able to be passed on to other offspring. Not all mutations are like this, it is an extreme few that can be out of the mutations that occur. Most of the time, more so in the past, people with genetic mutations would die early on in life, while the select few that got beneficial mutations were more likely to live on. This is natural selection. In this way, the best and strongest survive, making the entire species stronger.
The reason genetic disorders don't disappear and are so prevalent today is because of modern medicine. We are able to keep people with these disorders alive much longer than they would have survived on their own, often long enough to procreate and pass that disorder on. It is because of medicine that humans, as a species, are weakening. Ironic, isn't it? That a practice to save the individual is ultimately condemning the species.
Interestingly, some genetic disorders are still around because they can prove helpful in certain forms. For example, while sickle-cell anemia severely weakens those who have it, people who carry the abnormal gene but also have a normal copy are less likely to die of malaria, which is why natural selection never removed the mutation.
Yeah, it's an incomplete dominance thing, (or is it co-dominance?) with sickle cell anemia. If you live in Africa and have zero alleles for it, you are very susceptible to Malaria. If you have one allele for it, the red blood cells act normal until attacked by a Malaria worm, in which case they twist out of shape and strangle it. If you have both alleles for it, then your cells are just fucked. So yeah, I did mention above that there are some beneficial mutations (and all mutations are basically genetic disorders) that are beneficial, and so they are still around today.
I know this isn't based on scientific fact or even necessarily true but it made me smile and feel a little bit better about having to have such an illness/disability :)
left bottom corner the girl is making a weird face
the more you look the weirder it is