The way mono works in the body is tricky, so lots of people are confused about how long it is contagious. If you get mono, the virus stays in your body for life. That doesn't mean that you're always . But the virus can surface from time to time and risk infecting someone else.
Here's how it works:
Mono, or mononucleosis, usually is caused by an infection with the Epstein-Barr virus (EBV).
People who have mono can be contagious from the time they first become infected. But they may not know that they have the virus. It takes a while for mono symptoms (like tiredness, fever, muscle aches, headache, or sore throat) to show up — about 1–2 months, in fact. This is called the incubation period.
To make things even more confusing, some people can carry the virus without ever getting any mono symptoms. So they may not know they have been infected, but they can still pass it to others. In fact, most people have been infected with EBV by the time they reach adulthood.
People are definitely contagious while they have symptoms, which can last 2–4 weeks or even longer. Health experts aren't sure how long people with mono stay contagious after symptoms are gone, but it seems they can spread the infection for months after that. Then, the virus stays dormant (inactive) in the body for the rest of a person's life.
Sometimes the dormant virus can "wake up" and find its way into a person's saliva (spit). That person might not feel ill or show any mono symptoms, but can spread the virus to other people. So there's a very small chance that people who have had mono in the past can pass it to others, even when they feel OK.
The bottom line is that it's hard to prevent mono from spreading. Because EBV is so sneaky, infections are common. That's why doctors urge everyone to wash their hands well and often. It's the best way to keep germs at bay.