Five years after Covid-19 shut down activities all over the world, medical historians sometimes struggle to place the pandemic in context.

What, they are asking, should this ongoing viral threat be compared with?

Is Covid like the 1918 flu, terrifying when it was raging but soon relegated to the status of a long-ago nightmare?

Is it like polio, vanquished but leaving in its wake an injured but mostly unseen group of people who suffer long-term health consequences?

Or is it unique in the way it has spawned a widespread rejection of public health advice and science itself, attitudes that some fear may come to haunt the nation when the next major illness arises?

Some historians say it is all of the above, which makes Covid stand out in the annals of pandemics.

In many ways, historians say, the Covid pandemic — which the World Health Organization declared on March 11, 2020 — reminds them of the 1918 flu. Both were terrifying, killing substantial percentages of the population, unlike, say, polio or Ebola or H.I.V., terrible as those illnesses were.