Why is there no vaccine against infectious mononucleosis?

Finding a safe, effective “mono” vaccine has proved devilishly complicated – although the need is obvious: Every year, a new crop of college students comes down with mono, which causes a terrible sore throat, enlarged spleen and debilitating fatigue that can last for weeks. Spread by saliva (kissing, sharing beer bottles, etc.), mono is so ubiquitous that by adulthood, 90 percent of American adults have had it.