The skin and/or mechanical factors, such as coughing, ciliary action or urine flow, along with phagocytes and humoral and cell-mediated immunity, help to protect the body from infection. In compromised patients, these mechanisms are impaired or bypassed, rendering patients susceptible to infection by opportunistic organisms (bacteria, mycobacteria, fungi, viruses, parasites). Mechanical barriers may be breached in patients with burns or indwelling catheters, in diabetics or cancer patients, and in parenteral drug abusers. Humoral or cell-mediated immunity may be suppressed in non-neoplastic patients following splenectomy, in diabetics, in alcoholics and in those with certain rheumatic disorders, as well as in patients undergoing haemodialysis or receiving steroids or immunosuppressive drugs. In cancer patients infection is often a cause of death, frequently occurring as a result of granulocytopenia, obstruction of natural passages, or suppression or dysfunction of humoral or cell-mediated immunity. Diagnosis of the infection may be difficult and expert advice should be sought. Treatment should be started before a conclusive diagnosis has been made, particularly in neutropenic patients since infections may be life-threatening.