HIV stands for human immunodeficiency virus. HIV is a virus. Viruses infect the cells that make up the human body and replicate (make new copies of themselves) within those cells. A virus can also damage human cells, which is one of the things that can make a person ill.

Why HIV is so Dangerous?

The immune system is a group of cells and organs that protect your body by fighting disease. The human immune system usually finds and kills viruses fairly quickly.
What makes HIV so dangerous is that it attacks the immune system itself, the system that would normally get rid of a virus. It particularly attacks a special type of immune system cell known as a CD4 lymphocyte.
HIV has a number of tricks that help it to evade the body's defenses, including very rapid mutation. This means that once HIV has taken hold, the immune system can never fully get rid of it.

There is not a way to tell just by looking if someone's been infected by HIV. In fact a person infected with HIV may look and feel perfectly well for many years and may not know that they are infected. But as the person's immune system weakens they become increasingly vulnerable to illnesses, many of which they would previously have fought off easily.
The only reliable way to tell whether someone has HIV is for them to take a blood test, which can detect infection from a few weeks after the virus first entered the body.

When it Become Aids?

A damaged immune system is not only more vulnerable to HIV, but also to the attacks of other infections. It won't always have the strength to fight off things that wouldn't have bothered it before.

As time goes by, a person who has been infected with HIV is likely to become ill more and more often until, usually several years after infection, they become ill with one of a number of particularly severe illnesses. It is at this point in the stages of HIV infection that they are said to have AIDS . When they first become seriously ill, or when the number of immune system cells left in their body drops below a particular point.

There is still no way to cure AIDS. Using antiretroviral medication can slows the progression from HIV to AIDS, and can keep some people healthy for many years. In some cases, the antiretroviral medication seems to stop working after a number of years, but in other cases people can recover from AIDS and live with HIV for a very long time, although they have to take powerful medication every day of their lives, sometimes with very unpleasant side effects.