HIV infects the CD4 lymphocytes of the immune system. The CD4 lymphocytes are a special group of white cell which play an important role in protecting the body from infections. HIV introduces its own genes into the nucleus of the CD4 lymphocytes giving instructions to produce millions of new HIV. These HIV are then released into the blood stream where they infect and kill other CD4 lymphocytes. When HIV causes illness it is called symptomatic HIV infection or HIV disease.
HIV belongs to a group of viruses known as retroviruses. Retroviruses usually cause long periods of silent infection before symptoms and signs of disease appear.
Retroviruses contain a RNA genetic code. The viral enzyme reverse transcriptase allows HIV to make DNA copies of its RNA. The DNA copy is then inserted into the nuclear DNA of CD4 lymphocytes. The addition of this new information enables the virus to take over control of the CD4 lymphocytes and instruct them to produce huge numbers of new HIV. Only retroviruses have this ability to make a cellular DNA copy of their viral RNA code.