Money quote right here.But in the dark ages, travel between long distances was both slow and arduous, which played a huge part in helping to limit the spread of these super deadly variants, as the infected would have shown symptoms and either died or were murdered/quarantined before reaching new population centres.
These history wanna-be charlatans are often conveniently dont mention that travel was far far too slow in the past which limited the infection rates.
Also.lets not forget that those days the sick people (when an epidemic happened) were shunned from the society and left to die. They killed them, exiled them from their villages, or the village would stop all people from entering due to fear of diseases.
What we are experiencing now is obviously nothing to compare with the past as we have a dramatically better technology but we also have ultra fast travel and far more practises which increases the chances of pandemics (animal farming, mass farms, etc)