 To explain the reason for the development of frequent vaso-occlusive crisis in some patients with SCD, the investigators stepwise hypothesis is that, due to the sickling process and the consequent development of intestinal ischemia, patients with SCD, especially those with frequent vaso-occlusive crisis, are more likely to have altered microbiota than healthy individuals. If the normal microbiota is altered, the intestinal barrier will be compromised, rendering the patients more susceptible to recurrent bacteremia of intestinal origin. Since most SCD patients with vaso-occlusive crisis present without any obvious infections, it is likely that the compromised intestinal barrier facilitates an inoculum of bacteremia not high enough to cause overt infections but sufficient to activate neutrophils and elicit vaso-occlusive crisis. The first step towards testing this overall hypothesis is to test the sub-hypothesis that altered intestinal microbiomes correlate with increased number of activated neutrophils and frequent vaso-occlusive crisis in patients with SCD. 