HIV is a virus. HIV works by infecting the cells of living organisms. The infected cells will then replicate (make copies). The HIV then spreads. People with HIV are known to be HIV positive. HIV is a virus that gradually attacks the immune system, which is our body’s natural defence against illness. If a person becomes infected with HIV, they will find it harder to fight off infections and diseases. AIDS is a syndrome caused by the HIV virus. It is when a person’s immune system is too weak to fight off many infections, and develops when the HIV infection is very advanced.