Indeed it could but it's highly unlikely with modern technology for information and obviously better medicine and knowledge. We'd really have to fuck up badly for it to get that bad. It'll be interesting to see how countries can manage something this bad and like I said maybe it will cripple the whole damn thing enough for the worst to happen but for my own sanity I have to believe we are advanced enough at this point to manage something on the scale of the spanish flu and reduce the impact to a fraction. A vaccine will come far, far sooner than any in time in the past, including just a decade ago with the H1N1 scare. Fingers crossed cause if tens of millions die it will fuck us bad for the next half decade, some third world countries would be totally obliterated.