From immunity to the role of genetics, Nature looks at five pressing questions about COVID-19 that researchers are tackling. Six months and more than ten million confirmed cases later, the COVID-19 pandemic has become the worst public-health crisis in a century. Ewen Callaway, Heidi Ledford, and Smriti Mallapaty write in Nature that scientists have learnt how the virus enters and hijacks cells, how some people fight it off and how it eventually kills others. They have identified drugs that benefit the sickest patients, and many more potential treatments are in the works. They have developed nearly 200 potential vaccines — the first of which could be proved effective by the end of the year. But for every insight into COVID-19, more questions emerge and others linger. Among them: Why do people respond so differently? What’s the nature of immunity and how long does it last? Has the virus developed any worrying mutations? How well will a vaccine work? What is the origin of the virus?