Most HIV tests are designed to detect the antibodies that the body produced to fight HIV once infection has occurred, but do not test for the virus itself. It can take some time for the immune system to produce enough antibodies for these tests to be able to detect them, and this time period can vary person to person. Most people will develop antibodies within three months, but it can take longer, including up to six months in some cases. This time period is known as the "window period", when an individual may be infected with HIV, but may not yet test positive. HIV can be passed to other people during the window period. Therefore, it is important to get tested after this period has passed (about 6 months after you think you have been exposed to HIV).