Severe viral infection Severe cytomegalovirus infection 2017-12-17T22:31:35Z A typical mononucleosis syndrome consists of an acute febrile illness with an increase of 50 percent or more in the number of lymphocytes or monocytes, with at least 10 percent of the lymphocytes being atypical. Immunocompromised patients may experience more severe complications such as CMV retinitis. An unusually severe infection by cytomegalovirus.