Denis Burkitt, an Irish surgeon, traveled to Africa during World War II as a member of the Royal Army Medical Corps, and afterward he settled in Uganda to practice medicine. There he observed that a surprising number of children developed strange jaw tumors, a cancer that would come to be known as Burkitt lymphoma. Eventually, Burkitt sent samples of the tumor cells to Middlesex Hospital Medical School in London, where Michael Anthony Epstein, a pathologist, and his colleagues Yvonne Barr and Bert Achong examined them through an electron microscope.

Their findings — they noticed particles shaped like a herpesvirus, only smaller — were published in a landmark paper in The Lancet in 1964 and spurred the realization that this newly identified member of the Herpes​viridae family, subsequently named Epstein-Barr virus, was a cause of Burkitt lymphoma. It was the first evidence that a viral infection could lead to cancer. The virus has since been shown to increase the risk of Hodgkin lymphoma, as well as nasopharyngeal and stomach cancer. It is also the virus most often responsible for infectious mononucleosis, a disease usually characterized by extreme fatigue, sore throat, fever and swollen lymph nodes in the neck. These symptoms can last for weeks and, in chronic cases, recur for years.

We now know that upward of 90 percent of adults have the Epstein-Barr virus. As happens with other herpes​viruses, once you have been infected, the virus stays with you forever — it deposits its DNA alongside yours in the nucleus of many of your cells. (RNA viruses, like SARS-CoV-2, can be cleared from your body.) Most people contract Epstein-Barr in childhood: It is spread through body fluids, usually saliva; kissing is a frequent route of transmission (as may be the sharing of utensils). Young children, if they get sick at all, typically develop symptoms indistinguishable from those of a cold or flu; mono is more common when the first infection happens after puberty. “Most people never know they’re infected,” says Jeffrey Cohen, the chief of the Laboratory of Infectious Diseases at the National Institute of Allergy and Infectious Diseases.

The virus enters cells at the back of the throat and from there moves into B cells, a type of white blood cell that produces antibodies. In some B cells, the virus replicates, making proteins that the immune system can recognize and subdue. In other cells, though, it remains dormant. “It’s very stealthy,” Cohen says. Ultimately, as those infected B cells circulate throughout the body, they reach the back of the throat again. The virus awakens and starts producing proteins, which its host sheds, potentially spreading the pathogen to others, probably for several days each month. “The vast majority of people who are infected are passing it around,” Cohen says. “It’s shed in our saliva the rest of our lives.”