By letting people die. If covid came around in the 1200s, then everyone would have gotten it and ~3% of the world population would have died. Definitely suboptimal but nowhere near an extinction event.
Given COVID’s risk/age relationship and already lower life spans in that time period plus higher vitamin D exposure rates it’s quite likely rate of death would have been lower in that population.
The question was about a world without modern medicine. It's totally reasonable to speculate there would be a higher fatality rate without medical intervention keeping people from dying.