It has three phases:
Window Period (Asymptomatic Stage): After the virus enters into body, it takes about 3-6 months to be detected by a blood test. This period during which the presence of HIV antibodies are not detectable in the blood is called a window period. During this period, a person may pass on the infection to others unknowingly. Or the person may pass on the infection if he has already tested his blood within three months after the exposure and has found to be negative.
Symptomatic Stage: After the window period is over, the virus start attacking the body's immune system. At this stage, the presence of HIV in the body manifests with some symptoms in the form of many opportunistic infections like flu, fever, diarrhea or tuberculosis. Most of them get cured after treatment. But the disease goes on progressing in the body rapidly depending on the personal hygiene, habits and nutritional status of the infected person.
Full-blown AIDS Stage: At this stage, the virus has completely overridden the body's immune system and many opportunistic infections occur repeatedly, or together. Some people develop AIDS a few years after becoming infected with HIV. Some people can be infected with HIV for 10 years or longer without developing AIDS.