HIV is a virus which attacks the immune system
The immune system is the body’s natural defence that protects it against disease and ill health. HIV is a virus which weakens the human immune system. HIV stands for human immunodeficiency virus. If left undiagnosed and untreated HIV can cause damage to the immune system which can leave people vulnerable to other infections.
AIDS stands for acquired immune deficiency syndrome, and is not the same as HIV. It is unlikely that the term AIDS would be used these days, particularly in healthcare. Advanced or late-stage HIV is how this would be referred to. Even if someone becomes very ill with HIV, as long as it is diagnosed and treated in time for the immune system to recover, then usually the person can recover to some extent. Early diagnosis and treatment is the key to avoid this happening.
You cannot catch AIDS and there is no such thing as an AIDS test.