Mononucleosis aka "mono", caused by Epstein-Barr virus (EBV). Spreads through contact with saliva, mucus from nose/throat, tears, sharing eating utensils or toothbrushes. Treatment: rest, salt water gargle, Tylenol, or corticosteroids in severe cases. Avoid heavy lifting/contact sports as spleen may be enlarged and straining/impact could cause it to burst.
Atypical lymphocytes are shown here. These WBC's are "atypical" because they are larger (more cytoplasm) and have nucleoli in their nuclei. The cytoplasm tends to be indented by surrounding RBC's. Such atypical lymphocytes are often associated with infectious mononucleosis from Epstein-Barr virus (EBV) infection.