The background to this latest EASAC study is the debate about risks and benefits of research that modifies viruses to better understand their functioning, in particular their potential to cause pandemics. Special attention is given to viruses spreading from animals to humans and to human viruses spreading by aerosol route. Such research poses not only questions of biosafety, relating to the prevention of harm by accident, but also questions of biosecurity, meaning the intentional misuse of viruses made more potent in the laboratory.