An example in the same realm is sickle cell disease!
Sickle cell disease is a genetic mutation that results in red blood cells being sickle-shaped. If an individual receives this mutated gene from both parents, they will likely die an early age due to various complications. However, if they are simply carriers of the illness, having inherited the gene from one parent, but not the other, they will often have enough normal red blood cells to mask the effects of sickle cell disease.
This gene has been shown to help individuals resist malaria. Since the sickle-shape cells are weaker than their normal rounded-cell counterparts, the parasite often cannot hitch a ride in these cells. This gives the immune system more time to react to and get rid of the parasite!
For this reason, sickle cell disease is prevalent in areas where malaria is prevalent.
