Answering this question from my dear friend Yingxuan:
Why hasn't hereditary fatal illness been removed by natural selection since the ill should have died and pass down these defective genes?
After talking to my primary school friend who happens to be around, we came up with the following reasons:
1. With modern medical advancements, some of these people managed to live past their reproductive age and actually have offspring.
2. Some of these hereditary fatal illness eg some forms of cancer has actually developed due to the toxics,radiation that we experienced in the modern age.
However, these doesn't answer the question at its most basic level, because modern medical advancement and pollution only happened in the last couple thousand or even hundreds of years, surely with tens of thousands of years we should have eradicated these genetic illness through natural selection?
The reason could be that some of the genes that caused these genetic diseases don't get expressed normally - they stay as genotypes, however when 2 persons carry this same gene gives birth to a offspring - the gene may finally be expressed ( as phenotype) as developing into the genetic disease. Hence there may be lots of people out there who are carrying the genes that cause the genetic disease, but they never get expressed hence natural selection has never got the chance to remove them from our genetic pool.
The above is just musings of an amateur evolutionary biologist.. don't take it for real!