Infectious mononucleosis, or mono, sidelines high school athletes every year. Kids who play sports aren’t more susceptible to the disease. Instead, adolescent competitors are at a greater risk for one of the illness’s serious complications—splenic rupture. Mono starts with fatigue, sore throat, fever and swollen glands. By then, it’s invaded the lymphatic system, which can cause spleen enlargement. For athletes, that’s a danger because the trauma and...