Halacha actually gives us a definition of דבר בעיר. People have been quoting various sources about what to do in a time of דבר, but they ignore the definition. According to the halacha, דבר exists when on three consecutive days the death rate exceeds one death per 500 working-age men. So if we assume 1/4 of the population consists of men of working age, that would be one death per 2,000 people, or 50 per 100,000, each day.
Of course the current epidemic has not come anywhere near that. Which means the halachos of דבר do not apply, and through Hashem’s mercy what we are experiencing, however bad it is, is nothing like what our ancestors used to experience routinely every 15-20 years if not more often. May we never see such times, and may our current suffering be enough to atone for all our sins and to achieve whatever it is supposed to.