Statistically speaking, the virus known as Epstein-Barr is inside you right now. It is inside 95 percent of us. It spreads through saliva, so perhaps you first caught the virus as a baby from your mother, who caught it as a baby from her mother. Or you picked it up at day care. Or perhaps from a friend with whom you shared a Coke. Or the pretty girl you kissed at the party that cold New Year’s Eve.

If you caught the virus in this last scenario—as a teen or young adult—then Epstein-Barr may have triggered mono, or the “kissing disease,” in which a massive immune response against the pathogen causes weeks of sore throat, fever, and debilitating fatigue. For reasons poorly understood but not unique among viruses, Epstein-Barr virus, or EBV, hits harder the later you get it in life. If you first caught the virus as a baby or young child, as most people do, the initial infection was likely mild, if not asymptomatic. Unremarkable. And so this virus has managed to fly under the radar, despite infecting almost the entire globe. EBV is sometimes jokingly said to stand for “everybody’s virus.” Once inside the body, the virus hides inside your cells for the rest of your life, but it seems mostly benign.

Except, except. In the decades since its discovery by the virologists Anthony Epstein and Yvonne Barr in 1964, the virus has been linked not only to mono but also quite definitively to cancers in the head and neck, blood, and stomach. It’s also been linked, more controversially, to several autoimmune disorders.