HIV stands for Human Immunodeficiency Virus.
You can become infected with HIV from anyone who is already infected, even if they don't appear to be ill and even if they haven't yet tested HIV-positive. HIV is present in the blood, vaginal fluid, semen, and breast milk of people who are HIV positive and it can be passed on through contact with these bodily fluids by:
Getting a transfusion of infected blood used to be a way people contracted HIV, but now blood supplies are screened very carefully and the risk is extremely low.
Watch this film which explains what the HIV does inside your body.
AIDS stands for Acquired Immunodeficiency Syndrome. A syndrome means a collection of illnesses or symptoms, which means AIDS itself is not an illness. In the UK people generally do not refer to someone as having AIDS, and you can't actually contract 'AIDS', but you might contract HIV through one of the routes mentioned above.
If the HIV virus develops in the body without medication, and the body becomes weakened, a person may be given an 'AIDS defining' diagnosis. This person will have a particular infection that is linked to their HIV virus, and the damage to their immune system, such as pneumonia, or their CD4 cell count (the number of CD4 cells present, which is how the immune system is measured) will have has fallen below a certain number. This reflects the damage done by HIV to the CD4 cells which fight infections.
Before there was HIV medication being given an AIDS diagnosis would have been more worrying as it means a severe weakening of the immune system. However, with all the advances in medical science, people with HIV may become ‘AIDS defining’, but they can go on to become completely healthy again. In the UK, HIV is now seen as a manageable chronic condition.