DocM, on 16 July 2012 - 17:02, said:
Evolution is always a tradeoff; people with sickle cell trait are resistant to malaria, but those with sickle cell disease (a much smaller number) have a shorter life expectancy, often dying in their 40's. Malaria is the greater threat where the trait evolved, so passing it on is a net benefit.
Nature's a pretty harsh mistress in general. Of course not all evolutionary traits are beneficial but it's nice to be a beneficiary of the ones that are i'd wager.