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.