what is the cause of lymphocytosis?
Lymphocytosis - Causes. 1  Lymphocytosis is a feature of infection, particularly in children. 2  Causes of absolute lymphocytosis include... 3  acute viral infections, such as infectious mononucleosis [3] (glandular fever), hepatitis [3] and Cytomegalovirus infection [3] 4  other acute infections such as pertussis [3]