http://www.hivguidelines.org/

What is the “window period”? The “window period” occurs between the time of HIV infection and the time when diagnostic tests can detect HIV. The length of the window period varies depending on the type of diagnostic test used and the method the test employs to detect the virus. Can an infected person transmit HIV to others during the window period? Yes. During the window period, and despite a negative test result, an HIV-infected person may transmit the virus to others. How long can the window period last? The window period varies slightly from person to person, but virtually everyone infected with HIV develops antibodies within 3 months of infection.