The most common cause of high lymphocytes and monocytes is viral infections, such as influenza virus, rubella virus, hepatitis B virus, EBV, etc. When viral infections occur, the body’s immune response stimulates the proliferation of lymphocytes and monocytes, which appear in elevated numbers. At the same time, there is a clinical disease called infectious mononucleosis, which is caused by EBV infection. After infection with this virus, there is a significant increase in lymphocytes and monocytes, and there is also a kind of heterogeneous lymphocytes, that is, morphologically variant lymphocytes, and the percentage of such heterogeneous lymphocytes often exceeds 10% or even higher levels. Even after antiviral treatment and the patient’s symptoms have completely disappeared, the elevated heterogeneous lymphocytes remain in the peripheral blood for weeks or even months. Therefore, if there are high levels of lymphocytes and monocytes, the first thing to consider is a viral infection.