Step 1: Think about which pathogen exploits the deep immunosuppression that follows a renal transplant.
Step 2: That pathogen is cytomegalovirus. It is the leading opportunistic virus after kidney transplant and produces a recognisable syndrome of fever, low white cell count, enlarged liver and spleen, and aches in muscles and joints.
Step 3: Ordinary bacteria like Klebsiella, Streptococcus and Staphylococcus are not the signature post-transplant organism being asked about. The expected answer is CMV.
\[\boxed{\text{CMV}}\]