The immune system is the body's natural defense system that helps fight infections. An impaired immune system does not work properly and cannot effectively protect a person against infection.

Some conditions and medicines weaken or impair the immune system. These may include:

• Alcohol or drug abuse or withdrawal.
• Certain diseases or conditions. Diabetes, cancer, HIV/AIDS, or conditions in which the body mistakenly identifies its own tissues as harmful (autoimmune disorders) are some examples.
• Chemotherapy or radiation therapy.
• Some medicines, such as corticosteroids or those taken to suppress the immune system after an organ transplant.
• Surgery to remove the spleen (splenectomy).