For the vast majority of populations in the developed world, exposure to fungal agents is of little consequence. Even otherwise healthy individuals who require hospitalization will not succumb to fungal disease even if risk factors such as integument breaches through surgery and intravenous therapy are present. The major reason for this is the presence of an intact immune system that can respond to and kill or inactivate the invading organism.
However, in patients with compromised immunity, due either to underlying medical conditions or to medical treatment, harmless fungi can transform into opportunistic pathogens. The major players in the immune system needed to defend against fungal infections are the neutrophil and T lymphocyte, in particular the CD4+ helper cell. Although specific medical conditions or treatments may primarily affect either the innate or cell-mediated immune systems, a great deal of overlap exists.