HIV is a virus that attacks cells in the immune system, which is our body’s natural defence against illness. The virus destroys a type of white blood cell in the immune system called a T-helper cell, and makes copies of itself inside these cells. T-helper cells are also referred to as CD4 cells.As HIV destroys more CD4 cells and makes more copies of itself, it gradually weakens a person’s immune system. This means that someone who has HIV, and isn’t taking antiretroviral treatment, will find it harder and harder to fight off infections and diseases.If HIV is left untreated, it may take up to 10 or 15 years for the immune system to be so severely damaged that it can no longer defend itself at all. However, the rate at which HIV progresses varies depending on age, general health and background.