MSRA-0937

A 32-year-old man presents to his General Practitioner having been feeling unwell for a few days with a persistent cough.
Investigations:
Investigation Result Normal value
Haemoglobin (Hb) 130 g/l 120–160 g/l
White cell count (WCC) 18 × 109/l 4.5–11.0 × 109/l
Neutrophils 12 × 109/l 2.0–7.5 × 109/l
Platelets (PLT) 450 × 109/l 150–400 × 109/l
A blood film shows atypical lymphocytes.
What is the most likely diagnosis?