HIV infection occurs via the transfer of blood, semen, vaginal fluid, or breast milk from one person to another. Within these bodily fluids, HIV can be present as a free virus particle and/or as a virus within an infected donor immune cell. HIV infects and kills host leukocytes known as CD4 cells (also called T cells or T-helper cells). CD4 cells are essential to the body in fighting off infections. As these cells are lost, so is the body's ability to fight infection.
Possibly months after the initial infecting episode, an infected person develops a mononucleosis-like illness lasting a week or two. A person may then be free of symptoms for years; however, as the CD4 cells die, the person becomes vulnerable to many serious infections.
The Centers for Disease Control and Prevention (CDC) uses a stage system to classify HIV infection. HIV infection is classified as stage 3 (AIDS) when the immune system of a person infected with HIV becomes severely compromised (measured by CD4 cell count) and⁄or the person becomes ill with another serious infection. In the absence of treatment, AIDS usually develops 8 to 10 years after initial HIV infection; with early HIV diagnosis and treatment, this may be delayed by many years.
There is no vaccine available to develop specific immunity against HIV.