Inclusion Criteria:  Participants fulfilling all of the following inclusion criteria are eligible for the infectious mononucleosis (IM) group:  Informed consent as documented by signature Confirmed primary EBV infection as confirmed by the treating clinician and defined by: Compatible clinical (infectious mononucleosis symptoms including but not limited to malaise, headache, fever, tonsillitis, pharyngitis, cervical lymph nodes enlargement) and laboratory picture (lymphocyte count elevation, LUC cells, reactive lymphocytes in manual differential, elevated liver enzymes; of note, not all typically described features have to be fulfilled)  AND  serology compatible with primary EBV infection (anti-EBNA IgG negative, anti-VCA IgG negative, anti-VCA IgM positive OR anti-EBNA IgG negative, anti-VCA IgG positive, anti- VCA IgM positive).  Participants fulfilling all of the following inclusion criteria will be eligible for the control group:  Informed consent as documented by signature.  one of the following:  Clinical picture of upper respiratory tract infection (including but not limited to tonsillitis/pharyngitis, malaise, headache, cough, rhinitis, cervical node enlargement) confirmed primary Cytomegalovirus (CMV) infection (an optimal control group; however, the number of patients with a diagnosis of primary CMV infection is limited).  Exclusion Criteria:  Suspicion/diagnosis of IM as per judgement of the treating clinician (control group only); this individual may be eligible later for the IM group if primary EBV infection is confirmed, subsequently. Immunosuppression (broadly defined as primary/secondary immunodeficiency or treatment with an immunosuppressive medication including ≥ 10mg prednisone equivalent). History of autoimmune disease (e.g. SLE, vasculitis etc.)