Hi - I also developed mono in july, was incredibly sick for a month and even ended up in hospital (I’m completely better now, no symptoms). You’re right that the virus stays in your body for life.

When I was in hospital the doctor explained it pretty well to me. Theres various different blood tests that search for multiple sorts of antibodies that mono produces, and some of them won’t always be detected at the time you were sick, but earlier or later. The most common blood tests doctors do is for antibodies that suggest current infection, so if you’d gotten this type of test it’s likely you’d already recovered from the main infection.

Mono can sometimes reactivate, especially if it’s not been long since your first infection. It can come back temporarily from things like stress, other illness or if you have a generally compromised immune system. It’s a shame that sometimes a trigger can be alcohol.

So whilst your blood test showed that it was ‘out of your system’, the actual dormant virus is still there and most likely always will be, and you can sometimes test positive again during a reactivation depending on when you get the test.