Some conditions and medications weaken or impair the immune system, which is the body’s natural defense system against infection. Examples of conditions and medications that impair the immune system include:

• Alcohol or drug abuse or withdrawal.
• Cancer.
• Chemotherapy.
• Conditions in which the body’s immune system reacts to its own tissues as if they were foreign substances (autoimmune disorders).
• Corticosteroids.
• Diabetes.
• Infection with the human immunodeficiency virus (HIV).
• Intravenous (IV) drug use.
• Medications taken to suppress the immune system and prevent the body from rejecting an organ transplant.
• Surgery to remove the spleen (splenectomy).

People who have impaired immune systems have a higher-than-average risk of developing infection. Vaccinations and medications need to be considered to prevent infections in people with an impaired immune system.