The way mono works in the body is tricky, so lots of people are confused about how long it is contagious. Once someone gets mono, the virus stays in that person's body for life. That doesn't mean that you are always contagious if you've had mono. But the virus can surface from time to time and risk infecting someone else.
Here's how it works:
Mono is short for mononucleosis. It 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 with the virus. But they may not know that they have the virus at first. That's because it takes a while from the time someone gets infected to the time symptoms of mono (like tiredness, fever, muscle aches, headache, or sore throat) show up — about 4 to 7 weeks in fact. This is called the incubation period.
To make things even more confusing, some people can carry the virus without ever getting any symptoms of mono. So they might not even know they have the infection.
Mono needs to run its course naturally. Symptoms may last 2 to 4 weeks, and some people feel tired for several weeks longer (that's why it's important to take care of yourself and get lots of rest).
Doctors and researchers aren't exactly sure how long people with mono stay contagious after symptoms are gone. They believe that people can spread the infection for many months after their symptoms are completely gone — some studies show as long as 18 months. After that, the virus remains dormant (inactive) in the body for the rest of a person's life.
If you've had mono, the virus can find its way into your saliva at times. When that happens, it can spread to other people, even though you don't feel ill or show any signs of having mono.
The bottom line is that it's hard to prevent mono from spreading. That's why you'll want to wash your hands often and avoid sharing drinks and eating utensils with others.