Epstein-Barr Virus is the cause of heterophile-positive Infectious Mononucleosis.  Typical age range is 15-24 years old and transmission is via salivary secretions.  Incubation period is 4-6 weeks, and classic symptoms are preceded by a prodrome of fatigue, malaise, and myalgias for 1-2 weeks, which then morph into the typical features of severe pharyngitis, which may be exudative, low-grade fever, lymphadenopathy, and sometimes headache. The lymphadenopathy is often in the posterior cervical region and some of the nodes can be fairly large in diameter, which is a decent clue to diagnosis in the right setting.  Some patients treated with PCN derivatives prior to diagnosis, and thus, treated erroneously for Strep Pharyngitis, may develop a morbilliform rash, but this has been called into question.  The rash may just be attributed to EBV itself in 5% of patients, irrespective of PCN/Amoxicillin use.  Leukocytosis is often present, with a marked lymphocytosis, including > 10% atypical lymphocytes, and LFT’s are elevated in > 90% of cases.  Heterophile antibody test confirms the diagnosis and is present in 40% of patients during the first week of illness and in 90% at 3 weeks, so if initial test is negative early on, and suspicion remains high, retest in one week.  Disease is self-limited with supportive treatment.  Splenomegaly occurs in up 50% of patients and consensus is to withhold physical activity for at least 3-4 weeks due to the risk of splenic rupture, which occurs in 1% of patients and often necessitates splenectomy.  10% of IM cases are due to CMV which is heterophile negative.