Mono is usually caused by EBV (Epstein-Barr Virus), and once exposed to it/contracting it, it sticks with you for the rest of your life, whether or not you actually show symptoms. And generally, if you have it once, you won't have it again.
Being close to someone/breathing on them won't give you mono, though. You need to come in contact with saliva, either through kissing, or sharing of glasses, toothbrushes, eating utensils, etc.
It's much harder to contract mono through casual contact; I'm not sure what you mean about "lessened contagion", because the virus stays with you your entire life, so once you got it, you're stuck with it. You may or may not pass it on to anyone: most of the time the virus just goes dormant.
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