An explosive diarrhea-causing parasite has infected hundreds in upwards of 17 states since May 1, with a large number of cases reported in Illinois, according to the Centers for Disease Control and Prevention. The CDC and the Food and Drug Administration are working to determine a common thread among the illnesses, but a singularly related outbreak hasn't been identified. Instead, the cases thus far appear to be part of a seasonal surge, though officials are searching for a potential throughline.