Although they are often mixed up these two words have different meanings. 'HIV' is the name of a virus, whereas 'AIDS' is a name for a collection of illneses caused by this virus.
HIV
HIV stands for 'Human Immunodeficiency Virus’. ‘Immunodeficiency’ refers to how this virus weakens a person’s immune system, the part of the body that fights off diseases.
Some people notice no symptoms when they’re first infected with HIV. But within six weeks of infection most people suffer a short illness (lasting around two weeks) as their body reacts to the virus. This involves two or more of the following: body rash, sore throat or fever. Once this passes an infected person usually feels fine for a number of years. However, unless they start treatment before the virus causes too much damage, as years go by they will usually start to suffer life-threatening illnesses such as cancer, TB and pneumonia. This is because HIV is destroying cells (CD4 or T-cells) that our immune system needs.
HIV has been in humans for many decades but was only identified in the early 80s.
AIDS
AIDS stands for 'acquired immune deficiency syndrome'. It means a collection of illnesses (‘syndrome’) caused by a virus people pick up (‘acquire’) that makes their immune system get weak (‘immune deficiency’). You cannot get an AIDS diagnosis unless you are already HIV positive.
In the 1980s and early 90s HIV treatment wasn’t good at fighting the virus and most people with it were eventually diagnosed with AIDS. But now anti-HIV drugs can control (but not completely get rid of) the virus and far fewer people in Britain develop serious HIV-related illnesss. This means the term ‘AIDS’ isn’t used much by UK doctors now (instead they talk about late stage or advanced HIV disease or HIV infection).
Some people use ‘HIV’ and ‘AIDS’ as if they mean the same thing but, for example, there is an HIV test (but no ‘AIDS test’) and people can catch HIV (but they cannot ‘catch AIDS’).