If HIV isn't treated, the gradual weakening of the immune system it causes leaves the body vulnerable to serious infections and cancers which it would normally be able to fight off. These are sometimes called ‘opportunistic infections’ because they take the opportunity of the body’s weakened immunity.
If you develop certain opportunistic infections, you may be diagnosed as having AIDS. AIDS stands for acquired immune deficiency syndrome. Different people diagnosed as having AIDS may become unwell with different illnesses, depending on the opportunistic infections they develop. This is why AIDS is not considered a disease, but a syndrome – a collection of different signs and symptoms, all caused by the same virus, HIV.
AIDS is just a term that doctors and researchers sometimes find useful. You may also hear the term ‘late-stage HIV’ used to mean the same thing. If someone develops an AIDS-defining illness this doesn't mean they are on a one-way path to illness and death. With the right treatment and care, many people who have been diagnosed as having AIDS recover from their AIDS-related illness and go on to live long and healthy lives.