I don't have it in front of me, but Jared Diamond's Guns Germs and Steel discusses this. IIRC, sickle cell is to malaria as cystic fibrosis is to cholera as tay-sachs is to TB. I think lactose tolerance has more to do with culture than disease.
I'm pinging G3k because he always has something to say about this.
In the old areas of malaria, the mutation allowed the mutants to survive longer than the non-mutants who had no resistance to it. In the short term, it conferred a great benefit, in todays world, it is a miserable affliction.
Being heterozygous for hemoglobin-S still protects, being homozygous is real bad. Granted, American blacks don't get any benefit from it. Do a Google search on thalassemia, similar to sickle cell but found in Europe rather than Africa.