A patient has undergone a renal transplantation 2 months back and now presented with difficulty in  breathing. X-ray showed bilateral diffuse interstitial pneumonia. The probable etiologic agent would be :
Cytomegalovirus (CMV) is the largest virus in the herpes family. 
It is transmitted transplacentally from a latent mother to foetus. 
Postnatal infections are usually acquired by means of sexual intercourse, blood transmission and organ transplantation , clinical disease resembles infectious mononucleosis . 
It may cause hepatitis and pneumonitis. 
For treatment Ganciclovir is the drug of choice.

Ref : textbook of microbiology, C P Baveja, 5th edition , Pg no:446
CMV
Histoplasma
Candida
Pneumocystis carinii