When you first become infected, your body starts to produce antibodies to fight it off. You may feel ill for a while at this stage. This is often called 'seroconversion illness' because it coincides with the time that blood tests for antibodies to HIV become positive.
Once seroconversion has taken place, you may stay well for a number of years without treatment. But there will be some signs of damage being done to your immune system. And you are infectious during this period; even though you may not be aware you have HIV.
This damage can be detected by laboratory tests. Doctors will use two key tests at your regular clinic appointments: