I don't think you can compare today with hundreds of years ago, even 50/60/70 years ago. London was a tiny place at the time of the great plague and the population was tiny compared with today. Even during the smog it was different to today, plus of course you must include possibly a million or two people that are not accounted for in the official population counts.
I reckon if that plague occurred with the population of today, we'd see millions killed, if it weren't for the way they turn out vaccines these days.