Opportunistic Infections (OIs) still remain a major cause of morbidity and death in children
with either malignant or nonmalignant disease.
OIs are defined as those infections occurring due to bacteria, fungi, viruses or commensal organisms
that normally inhabit the human body and do not cause a disease in healthy people, but become
pathogenic when the body's defense system is impaired. OIs can also be represented by unusually
severe infections caused by common pathogens. An OI could present itself at the onset of a
primary immunodeficiency syndrome as a life-threatening event. More often, OI is a therapyassociated
complication in patients needing immunosuppressive treatment, among long-term hospitalised
patients or in children who undergo bone marrow or solid organ transplantation.
The aim of the present review is to provide a comprehensive and ‘easy to read’ text that briefly
summarises the currently available knowledge about OIs in order to define when an infection
should be considered as opportunistic in pediatrics as a result of an underlying congenital or acquired