What do scientists mean when they say things like "after covid, we will never go back to normal"? Are they saying we must wear masks and socially distance in our day-to-day lives even after we are vaccinated for covid? If so, why is this? Why must covid permanently change human lifestyles forever, even though deadlier diseases like Spanish Flu and the Black Death did not (or at least they didn't stop the practice of socialisation and gatherings)?