The recreation of one of the deadliest diseases known could help us to prevent another pandemic. Or it might trigger one, say critics. Andreas von Bubnoff investigates whether the benefits outweigh the risks. Based on the latest sequence information (see ‘Flu viruses decoded’, below) researchers have reconstructed the deadly 1918 flu virus, and infected mice with it. Nature investigates whether the benefits of the work outweigh the risks. See also Nature's web focus: → www.nature.com/nature/focus/1918flu