Why would i have accute mono/ebv currently when i've had it before in the past? why would it re-activate?

May recur: Generally, people will only contract mononucleosis once. Once infected, the patient carries the dormant virus in b lymphocytes for the rest of life. Immune system normally keeps the virus under control. However, under certain circumstances when the immune system is under stress, the virus may become reactive and cause vague symptoms of general illness and person becomes contagious.