Most of the time, the immune system protects the body from germs and other threats. But sometimes it gets off track and has a weaker response to these threats. This low activity is called immune deficiency and it makes you less able to fight off infections.
It can result from medications or illness. Or it may be present from birth -- a genetic disorder known as primary immune deficiency. Examples include:
Severe combined immune deficiency (SCID). A genetic condition causing severe impairment in multiple areas of the immune system. Babies with SCID die from overwhelming infections, usually before reaching age 1. Bone marrow transplant can cure some cases of SCID.
Common variable immune deficiency (CVID). Due to a genetic defect, the immune system produces too few antibodies to effectively fight infections. Children with CVID typically have frequent infections of the ears, lungs, nose, eyes, and other organs. Treatment includes replacing the missing antibodies with regular injections of antibodies, called immunoglobulins.
Human immunodeficiency virus / acquired immune deficiency syndrome (HIV/AIDS). HIV infects and destroys immune system cells that normally fight infections. As the number of immune system cells declines, a person's vulnerability to infections rises steadily.
Drug-induced immune deficiency. Medicines that suppress the immune system result in an increased chance of infection. People taking immune-suppressing drugs for long periods require careful monitoring to detect and treat any infections that occur.
Graft versus host syndrome. After bone marrow transplant, the donor's immune system cells may attack the tissues of the person receiving the transplant. Prednisone and other immune-suppressing medicines are used to prevent excessive organ damage caused by the donor's immune cells.