How is the sickle cell trait affected by natural selection?

When you are heterozygous or have only one gene for sickle cell, you have resistance to malaria.  That was more important than the slight degradation to the carriers or the serious disease to those with both genes.  

I would think that Huntington's is/was/should be affected by natural selection.

Natural selection means that nature will weed out weak organisms and the strong organisms will survive. Sickle cell is a trait that causes the blood cells to be shaped like sickles, making it hard for the blood to squeeze through tiny spaces. Those people have less of a chance to survive than people with normal blood cells. Therefore, natural selection controls the sickle cell trait.

Huntington's disease develops later on in life, past the time where people have reached an age to reproduce. After an organism has successfully reproduced, anything can happen to them (humans - heart attack, getting hit by a truck, animals- not being eaten by a lion, not being infected by a virus...) and natural selection won't play a role anymore.

I don't know about Huntington's, but I think I know why sickle cell is still around. It is most common in Africa and people who's ancestors came from Africa, and I'm not being racist it's just a fact, and the reason why natural selection hasn't eliminated it is because it is believed people with sickle cell anemia are less likely to get malaria. This being said Malaria is one of the most common causes of death in Africa, so people who have sickle cell are more likely to survive. Also not everyone who carries the sickle cell gene has sickle cell, so even though they might not have it, it is possible their kids can, carrying on the trait.