This book presents the morphological details of various  opportunistic pathogens for prompt identification, which is essential for  the proper management of various bacterial, viral, fungal and parasitic  infections encountered in immunocompromised patients. Over the past few decades, certain therapeutic advances  such as  organ transplantation, prolonged use of steroids and other immunosuppressants  for various diseases and chemo-radiotherapy for the treatment of malignant  tumors  have, in addition to improving patient survival, also contributed to  significant aberrations in the patients  host defense mechanisms. Further, due  to compromised host resistance, certain clinical conditions such as HIV/AIDS,  diabetes mellitus, and major organ-related problems like chronic renal failure,  chronic liver and chronic lung diseases etc., make these patients potentially  vulnerable to a variety of opportunistic  infections.  The challenges posed by  the opportunistic pathogens are enormous.