The human immunodeficiency virus (HIV) works by attacking the human immune system. HIV cannot replicate on its own, so in order to make new copies of itself, it must infect cells of the human immune system, called CD4 cells. CD4 cells are white blood cells that play an essential role in responding to infections in the body. CD4 cells are killed by HIV and the body’s ability to recognize and fight some types of infection begins to decline. If HIV is not controlled by treatment, the loss of CD4 cells leads to the development of serious illnesses.