Immunocompromised hosts remain exposed to normal community acquired pathogens and are at risk of developing opportunistic infections. The pathogens involved are by and large the same as those affecting immune competent hosts. Some specific pathogens unique to patients with compromised cell mediated immunity include Listeria monocytogenes, Nocardia spp, Pneumocystis jirovecii, cytomegalovirus (CMV), Cryptococcus neoformans, Aspergillus spp, Strongyloides stercoralis, etc.

SOT recipients are at risk of acquiring infections from donors with active or latent infections at the time of harvesting the organ. The organ being transplanted is a critical determinant of the location of infection in the immediate post-operative period. The chest, abdomen and urinary tract are the most common sites of infection in recipients of thoracic, liver and kidney transplantation, respectively. Intra-operative factors including events during surgery also contribute to infectious complications. Four weeks after transplant, immunosuppression is the major risk factor predisposing to infection.

The time of infectious event relative to the time of surgery may have an important bearing on the type of infection. Early infections (0–30 days after transplant) are usually associated with preexisting conditions or complications of surgery. Bacteria and yeast are the most frequent pathogens. Donor-derived infections usually present during this period. The intermediate period (31–180 days after transplant) is usually complicated by infections due to latent pathogens from donor organs, blood products, those re-activated in the recipient and the classical opportunistic agents. In the later period (beyond 180 days after transplantation), infection risks depend on immunosuppression and exposures.