Several versions are known to trigger common colds and more recently two types have set off outbreaks of deadly illnesses: severe acute respiratory syndrome (Sars) and Middle East respiratory syndrome (Mers).The questions are therefore straightforward: what have we learned over the past five months and how might that knowledge put an end to this pandemic?Then when these bat viruses move into other mammals, creatures that lack a fast-response immune system, the viruses quickly spread into their new hosts. “A Covid-19 infection is generally mild, and that really is the secret of the virus’s success,” adds Ball. “Many people don’t even notice they have got an infection and so go around their work, homes and supermarkets infecting others.” Even worse, in some cases, a person’s immune system goes into overdrive, attracting cells to the lungs in order to attack the virus, resulting in inflammation. “That is in line with other coronaviruses that infect humans,” says Skinner. “That means that even if most people do eventually become exposed to the virus, it is still likely to become endemic – which means we would see seasonal peaks of infection of this disease. Among the projects that are under way is a vaccine programme that is now in phase-one trials at Oxford University, two others at US biotechnology corporations and three more at Chinese scientific groups. As a result, some scientists have proposed a way to speed up the process – by deliberately exposing volunteers to the virus to determine a vaccine’s efficacy. “This approach is not without risks but has the potential to expedite candidate vaccine testing by many months,” says Nir Eyal, a professor of bioethics at Rutgers University.