Noted cardiac surgeon and Chairman of Bengaluru-based Narayana Health, Devi Prasad Shetty, said on Wednesday that the world has turned upside down due to the Covid-19 pandemic, and when historians would pen the history of the world 100 years from now, there would be two distinct periods, one is B.C., i.e., Before Covid, and the second is A.C., meaning After Covid.