Step 1: The key is the type of immune defect after a kidney transplant - drug-induced suppression of cell-mediated immunity, which uncovers latent viruses. Step 2: The classic opportunist that emerges is $Cytomegalovirus$, the most frequent viral infection of renal transplant recipients, peaking around 1-4 months post-transplant. Step 3: Recognise its syndrome: fever as the leading sign, plus leukopenia, hepatosplenomegaly and myalgias/arthralgias. Step 4: The bacterial options (Klebsiella, Streptococcus, Staphylococcus) are ordinary nosocomial bugs, not the signature transplant pathogen, so CMV is selected.
\[\boxed{\text{CMV}}\]