I doubt this. If majority of population had severe incurable diseases then humanity wouldn't survive, and healthcare would collapse. I think (sorry, no statistics) that majority of population are healthy enough to do hard physical work.
I never said the majority of the population has severe incurable diseases. I said that have genes that aren't great (maybe they have a predisposition for certain diseases or they're a carrier but are healthy themselves).
Also, many diseases don't effect you until after you're past reproductive age. Evolution doesn't care if you get Alzheimer's at 70 but it still has a fairly large burden on our modern day society.
Sickle cell is particularly interesting because the gene that causes it makes the carriers less susceptible to malaria. So there actually was an evolutionary pressure for people in areas with high levels of malaria to have it (which is why it's so common among African Americans). Apparently it's better to die of sickle cell at 40 after you've had kids then die of malaria when your 10.
