Well, first off - sickle cell is not a 'disease' - it's a genetic trait.  As hemophilia [bleeding w/o clotting], the slightly reduced immuno-responses of most Europeans [w/neanderthal genes], and CPT1A all are.  
I can see how it might have had a positive benefit in preserving needed body insulation in cold climates - and I can see how it might help prevent some of the negative consequences of that extra degree of body fat.  And, one of the more brutal facts of group/community survival - it may have been advantageous that more infants and toddlers died during periods of famine, when their mothers' lost their milk from hunger.  It would have been most essential to keep the hunters, the women capable of bearing future children, and the youth old enough to help out alive - for the group as a whole to survive past the hard times. Such a great sadness - I'm very glad they've figured this out and how to prevent it, now that it's no longer a potentially needed survival trait