HIV has no own metabolism. It is dependent on the human cell to replicate. HIV harms the immune system primarily by affecting the T-helper cells (CD4-positive cells).
A main function of the immune system is to defend the body against invading bacteria, viruses or fungi. In addition the immune system detects and destroys cancer cells. In advanced stages of HIV-infection the number of T-helper cells is reduced continuously. Therefore, the immune system becomes less capable of protecting the body from infections.
When the immune system is impaired, pathogens may lead to life-threatening diseases which are usually not seen in healthy immunocompetent individuals. These diseases are called "opportunistic infections"because they take advantage of the deficiency of the immune system.
When the CD4 cell count is seriously reduced and opportunistic infections or malignancies, such as Non-Hodgkin lymphoma or Kaposi-sarcoma develop the stage of aquired immuno-deficiency syndrome (AIDS) is reached.
In the past reaching AIDS was an irreversible process leading to death in the near future. Today, due to effective therapeutic options, the reversion even of this stage of severe immunodeficiency back to a mostly normal immune system is the norm. An almost normal life expectancy under antiretroviral therapy is the consequence.