Human Immunodeficiency Virus (HIV) Infection

The infection is caused by the human immunodeficiency virus (HIV).

After HIV is in the body, it attacks and destroys CD4+ cells, which are the part of the body's immune system that fights infection and disease. When HIV weakens or destroys the immune cells, it may lead to certain illnesses or diseases, such as some types of pneumonia or cancer that are more likely to develop in someone who has a weakened immune system. These conditions are a sign that HIV has progressed to AIDS.

HIV is spread when blood, semen, or vaginal fluids from an infected person enter another person's body, usually through sexual contact, from sharing needles when injecting drugs, or from mother to baby during birth.

HIV is rarely spread by blood transfusions or organ transplants in the United States because of improved screening procedures.