Why is it difficult to get rid of AIDS virus out the human body?

Actually getting rid of any virus is difficult. In order for a virus to be destroyed, your body's immune cells need to form antibodies that can specifically identify the virus and cause the virus and all the infected cells be neutralized or killed by other immune cells.

The problem with HIV is several:

First, the protein shell that surrounds the virus and makes it identifiable is constantly changing, so by the time an antibody is formed, it may not recognize the virus any more.

Second, the virus is a RNA retrovirus, meaning once it infects a cell, it can get itself copied into the DNA of the cell and hide there without detection, sometimes for years.

Third, the cells that the virus likes to attack are the very immune cells that are supposed to be fighting off the virus. This prevents the whole immune system from working the way it is supposed to and the virus just continues to spread unchecked.

Fourth, because it is a virus, normal antibiotics just don't work against it. And even the anti-virals that have been developed only slow down its ability to reproduce. They can't stop it completely or kill it because the virus is already a part of the cell's DNA. It continues to change and over time can become resistant to the meds and start reproducing again.

Because its hardwearing

Because the disease nearly nullifies your capacity to fight any disease. It's Acquired Immunization Deficiency Syndrome.

Because the virus replicates your own RNA (or is it DNA... can't quite remember...), so your body can't tell the difference between your normal cells and the virus. So the immune system doesn't fight the virus anymore. The virus just keeps on replicating and reproducing until your immune system just shuts down.

That was the overly sipmlified version. Just type AIDS or Acquired Immunodeficiency Syndrome into any search engine and you'll get a more complicated definition.

In addition to the reasons stated above, HIV is a retrovirus with a very high mutation rate. The virus changes about 1000 times faster than human nucleic acids. The human body usually does a fairly good job of keeping the virus down for a while, but eventually HIV will outrun the white blood cells, changing so fast that they cannot recognize it.

This is the main reason why vaccines are not a very promising solution to HIV. Just like last year's flu shot won't work well this year, a single HIV shot probably wouldn't work for anything except the virus particles from a single person, just because the virus changes so quickly.

HIV can be slowed down a lot with anti-retroviral products (Think TAMIFLU) but so far they have not kept it down long enough for the body to destroy the infection entirely.

The virus destroys the cells that would direct an attack on the virus.