The chronic infections brucellosis and syphilis may occassionally cause an atypical lymphocytosis. The disease is generally self limited and is caused by an Epstein-Barr virus infection of B lymphocytes. The atypical lymphocytes are large and reactive with abundant basophilic cytoplasm. Some may appear almost blast-like. These changes are not unique to IM, but can be seen in other viral infections.
Infectious mononucleosis is most common in adolescents and young adults.
Splenomegaly is common. CMV occassionally causes a lymphocytosis with symptoms similar to IM and may follow blood transfusions. Other viral causes of lymphocytosis include infectious lymphocytosis and HTLV-1 related transient lymphocytosis. This is mainly a disease of children and is benign. It may be related to coxsackievirus A or B6, echovirus, and adenovirus 12.