What is 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. 

HIV has been in humans for many decades but was only identified in the early 80s.

What does the virus do?
Some people notice no symptoms when they are 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 
• fever

Once this passes an infected person usually feels fine for a number of years.

AIDS-defining illnesses:
Unless they start treatment before the virus causes too much damage, as years go by they will usually start to suffer life-threatening illnesses (sometimes called 'AIDS-defining') such as:

• cancer
• tuberculosis (TB) 
• pneumonia

This is because HIV is destroying cells (CD4 or T-cells) that our immune system needs to protect us from infections.

What is 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 1990s 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 illnesses.

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.

What is the difference between HIV and AIDS?
Some people use the terms ‘HIV’ and ‘AIDS’ as if they mean the same thing but they don’t.

HIV is a virus and people with it have ‘HIV infection’. Most of them don’t have AIDS.

AIDS is a name to describe a set of illnesses people with HIV eventually get if they don’t receive treatment.

But treatment is so good that few people with HIV in the UK now develop AIDS.

There is a test for the virus (an HIV test) but there is no ‘AIDS test’.

And people can get HIV but they cannot ‘catch AIDS’.

Source : Terrence Higgins Trust