There's a good reason why lots of people are confused about how long mono is contagious. That's because the way mono works within the body is tricky.
Mono is short for mononucleosis. It is caused by an infection with the Epstein-Barr virus (EBV), a virus in the herpes family of viruses.
Once someone gets mono, the virus stays in that person's body for life. But this doesn't mean that if you've had mono you are always contagious. Over time, the virus becomes less contagious. Eventually, it's very unlikely that a person who had mono will transmit the virus to someone else.
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 in its early stages. That's because it takes a while from the time a person is infected to the time symptoms of mono show up — about 4 to 7 weeks in fact. (This is called the incubation period.)
To make it even more confusing, some people can carry the virus without having any symptoms of mono, so they might not know they have the infection at all.
Mono needs to run its course naturally — symptoms may last 2 to 4 weeks — and some people feel tired for several weeks longer. It's important to take care of yourself if you have mono and get lots of rest.
Doctors and researchers aren't exactly sure how long someone with mono stays contagious after symptoms are gone. But it's generally believed that a person can spread the infection for many months after the symptoms are completely gone — some studies show as long as 18 months. But after that, it's very unlikely that a person will give someone else mono.
Reviewed by: Nicole Green, MD
Date reviewed: October 2009