The World Health Organisation declared Covid-19 a Public Health Emergency of International Concern on January 30, 2020, when the illness was a pneumonia of unknown origin that was spreading in China. About six months later, more than 17 million people have been infected with the disease in at least 213 countries and regions and over 650,000 have died. While scientists now know a great deal more about Covid-19, plenty of unanswered questions remain.