There are two different tests that are performed to determine whether there has been exposure to the HIV virus. The first test is the EIA, or the ELISA test. These tests aim to find whether there are enzymes in the bloodstream that indicate exposure to HIV. These are tests done in 20 minutes or less. Once they come out negative, you should be OK. However, as with any virus, the time it takes to develop in the bloodstream varies from person to person, so it is recommended that one takes 2 tests per year (every six months) if you are an at-risk person.
The second test that is performed is only done when the ELISA comes out positive. This test is called the Western Blot and it uses more samples to find actual antibodies present. If there are antibodies it means that your body is already trying to defend itself against the exposure. This would mean that, yes, one is infected.
However, until continuous testing is consistently negative and the individual does not engage in risky situations, then it would be for sure that the tests will be, and continue to stay, negative.
Yes,a negative HIV test means "not infected" because test is based on the antibodies that body generates.. So body reacts very quickly to these viruses as they are on prowl over the lymph glands etc..But the infected person may not show the symptoms quickly depending on the resistance..