It is possible for EBV infection to cause mononucleosis. The most common laboratory finding of EBV infection is lymphocytosis, defined as an absolute count greater than 4,500/μL or a leukocyte sorting count of greater than 50% lymphocytes on a peripheral blood smear. Smears may also recognize significant heterogeneous lymphocytosis, defined as more than 10% of the total lymphocyte count. The majority of the patient’s reactive lymphocytes were CD8+ cytotoxic T cells. The patient’s total leukocyte count averages 12,000-18,000/μL, but may be much higher. Some patients have mild relative or absolute neutropenia and thrombocytopenia. In addition, besides EBV infection, mononucleosis is also seen in infectious diseases such as typhoid fever, tuberculosis, and malaria, as well as in certain hematologic diseases such as monocytic leukemia, lymphoma, and Hodgkin’s disease. To summarize, it is recommended to seek prompt medical attention to find out if the mononucleosis is caused by EBV infection.