The period of time between the entry of a virus into the body and the production of sufficient blood to allow detection of viral markers by testing methods is called the window period of viral infection. The length of the window period for each virus is determined relative to the detection reagent used. In the case of HIV, for example, the current window period for third-generation testing reagents is 6-8 weeks after patient exposure, while the window period for fourth-generation HIV testing reagents is 4-6 weeks after exposure. The window period varies depending on the sensitivity of the reagents, and with the increased sensitivity and the use of antigen detection and nucleic acid detection, the window period for the virus will be greatly reduced in the future.