HIV is a virus. When someone becomes infected with HIV the virus weakens and damages their body’s defence system (the immune system) so that it cannot fight off infections.
Someone who has HIV is diagnosed as having AIDS only when their immune system cannot cope and they develop one or more particular illnesses.
Most people with HIV, who are taking treatment, will not go on to develop AIDS. The term AIDS is not used very often now. Late stage or advanced HIV infection is used instead.