Among genetically identical mice infected with the same lethal virus, some live and some die — and researchers at Hokkaido University have traced that difference to a single, fleeting window of time. In the first 24 hours of infection, a burst of type I interferon either sets the immune system on a path toward survival or fails to do so, functioning as a biological checkpoint that may have gone unrecognized until now. The discovery, published in iScience, offers a possible answer to one of modern medicine's most vexing questions: why identical exposures produce such radically different fates.