Certain health conditions and medicines weaken the immune system's ability to fight off infection and illness. Some examples in children are:

• Diseases such as diabetes, cystic fibrosis, sickle cell disease, and congenital heart disease.
• Steroid medicines, which are used to treat a variety of conditions.
• Medicines taken after organ transplant.
• Chemotherapy and radiation therapy for cancer.
• Not having a spleen.