The human immune system is capable of protecting individuals from infection by a variety of bacterial, protozoal, fungal, and viral pathogens. However, when the immune system is weakened by age or disease (e.g., HIV infection) or by medication (corticosteroids, chemotherapy) or by treatments to prevent rejection in organ or bone marrow transplantation patients, these pathogens that normally do not cause clinic disease can cause infections. The common opportunistic pathogens are fungi, Mycobacterium avium cellulare, viruses, particularly cytomegalovirus infection (CMV), and Pneumocystis carinii. Patients with HIV infection, organ and bone marrow transplants are particularly vulnerable to opportunistic infections.
The immunosuppressed individual is vulnerable to both endogenous and external organisms. Opportunistic infections can result from exogenous acquisition of a particularly virulent pathogen (eg, meningococcal meningitis or pneumococcal pneumonia), reactivation of an endogenous latent organism (eg, herpes simplex virus (HSV), herpes zoster virus (HZV or shingles), or tuberculosis, and endogenous invasion of a normally commensal or saprophytic organisms (eg, bacteria, viruses, fungi, or protozoa/parasites). The exact type of opportunistic infection that occurs depends on the type and extent of immunologic alteration, whether it be cellular, humoral, phagocytic, or a combined defect; and on organisms present in the internal and external environments.
Opportunistic infections are often lethal despite treatment with anti-viral, anti-fungal or antibiotic medications. Therefore, there is a need to develop methods to strengthen the immune system of immunocompromised individuals to both treat and prevent opportunistic infection.