How/why did sickle cell anemia increase in frequency in the human population?

The character of sickle cell anaemia does not get fixed in the population Because it has more disadvantages than advantages

Explanation:

Any character will get fixed in the population if the character provides the progeny certain advantage to reproduce better. The rate of malaria infection is low, but such people may have breathing difficulties and organs may fail due to the wrinkled RBC's not able to flow smoothly in the arteries and hence causing blockages. the character may be recessive but again the amount of oxygen carried per healthy RBC is less and that makes breathing difficult. so as a character its a disadvantage for the person who is a career and hence not being selected to be fixed.