In transfusion-transmitted diseases, the time between the actual infection and when that infection is detectable by laboratory methods. The window period is often confused with the incubation period (the time between exposure to an organism and development of symptoms). Window periods vary by organism, as well as by technology used to detect that organism. For example, the HIV window period, when using enzyme immunoassay tests to detect antibody against HIV, is about 20-22 days. The window period drops to about 9-10 days when using nucleic acid testing to detect HIV genetic material directly. Blood centers are concerned with the latter part of the window period because it is generally the time of greatest risk of undetected transmission of an organism. “Window period” transmission of viruses remains a possibility, but it is a very tiny possibility.