The immune system is a group of cells and organs that protects you body by fighting diseases. Usually, the human immune system finds and kills viruses quickly and effectively.
So if the body's immune system attacks and kills viruses, where's the problem?
Also the common cold for example is caused by a virus.
Different viruses attack differents part of the body.
What makes HIV so dangerous is the fact that it attacks the immune system itself - the very organ that would normally get rid of a virus. In consequence, the body of an HIV infected person is not able to fight the disease.
There isn't any way to tell if someone is infected by HIV just by symptoms and appearance. The only reliable method to test someone for HIV is taking a blood test, which can detect the virus from a few weeks after the infection.
In fact, a person infected with HIV may look and feel perfectly well for many years and may not even know that he is infected. But as the person's immune system weakens, he'll become increasingly prone to illnesses which his body previously would have fought off easily.