Can you tell me how is the sickle-cell trait resistant to malaria?

Good question: Recently it was shown that sickle-cells can actually "infect" Plasmodium falciparum (common cause of malaria) with a microRNA segment that inhibits the growth ability of this organism. People with sickle-cell trait do this without suffering severe symptoms associated with sickle-cell disease. http://microbewiki.kenyon.edu/index.php/Malaria_Resistance_and_Sickle_Cell_Trait

Sickle Cell trait: In a person who has sickle-cell trait, the red blood cells are destroyed prematurely before the Plamodium can reproduce. According to one study “Sickle cell trait provides 60% protection against overall mortality. Most of this protection occurs between 2-16 months of life, before the onset of clinical immunity in areas with intense transmission of malaria.”