My boyfriend was recently diagnosed with mono, the day before he was diagnosed i did kiss him, should i be worried? what can i do?

look for symptoms: Mono is a viral illness that presents in young folk with a sore throat, fever, fatigue, aches and pains. It is usually self limiting but may have severe complications, splenic rupture, hepatitis. If you feel ill see your doctor.

Not much: Most adults had mono as kids as a minor viral illness & are immune for life.I assume your BF was contagious when you kissed,as most are highly contagious for 2-3 wks after diagnosis & likely before.This viral illness takes several weeks to become symptomatic. It is much more likely to cause a significant illness in adults than young kids but there is nothing you can do to prevent it if susceptible