If a disease like covid had come along prior to the twentieth century, it's very likely that we wouldn't even know about it today. A shrewd historian might be able to sus out the increased mortality through census records, land usage, serf wages, etc. but most likely it would go unnoticed (not trying to downplay our current situation, please wear your masks).

I think modern first worlders like us have a real difficulty conceiving of the level of suffering that would result from one of the pandemics that history does vividly remember. During the black death half the people in Europe died, most of them left to rot in their own homes or wherever they keeled over. Towns and villages disappeared, vast swaths of land went fallow, and many local governments and authorities vanished. After a few years of suffering there was a new social mobility and wages rose for the survivors, and what was left of the ruling class passed new laws to force the serfs back into their proper place. Even the black death didn't come close to the centuries of turmoil that the bronze age collapse unleashed on the western Mediterranean