Djino Parker had a swollen spleen six years ago. His mother, Erica Finkelstein-Parker, remembered those days and the resulting hospitalizations. Though Djino mostly recovered, a recent incident brought back old memories and a score of new terrors. A few months ago, as Djino, 16, was walking by his mother she noticed something strange about his shirt: The horizontal stripes weren’t lying flat across her son’s stomach. “I thought, ‘Oh God,’ when I looked at him,” Finkelstein-Parker said, speaking by phone from her home in Littlestown, Pennsylvania. She immediately called the doctor and said her son’s spleen was swollen and that...