Well in this article i will only try to give you a simple reason behind the cause of this disease.

Sickle Cell Anemia is a kind of disease caused by mutation mostly found in africans. In this disease the red blood cells changes its shape from round to sickle or comma shape. in this disease the blood cell capacity to hold oxygen reduces which cause other deadly effects.

Its a kind of adaptation by the human body in which a mutation occurs in the hemoglobin gene. The reason behind this adaptation is the malaria parasite "Plasmodium Vivax" this parasite directly infects the red blood cells. So the body adopted itself against the parasite and it changes the shape of red blood cells so that the parasite can't live inside the red blood cells, so it can be sayed that its a kind of adaptation which automatically takes inside the body and we can't able to understand.

So in last we can conclude that its a kind of disease which occurs due to gene mutation against a parasite.

-Regards

Shubham rana