system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Malaria and sickle cell anemia and why natural selection hasn't eliminated sickle cell disease.
gpt
If you are a carrier of sickle cell, malaria has difficulty properly infecting you, and even when infected you recover quicker and with less symptoms.

You survive a malaria outbreak with others, you all do what survivors do, 9 months later babies born with a pair of the carrier gene express sickle cell and are sick.  

Natural selection selects for sickle cell because sickle cell is a survival trait.  You see similar for Cystic Fibrosis in Caucasian populations where there's a high level of certain toxins which a CF carrier can deal with due to some unique mucosa/gut traits.

Put 2 carriers together, and you have someone with a terrible disorder.