Pandemics always coincide with warfare because of the mass movement of people that takes place at the same time. It's probably somebody coming from an area infected by a biological military weapon into a civilian city aboard government transport who will start it.
The victims of the 1918 pandemic may have died primarily from infection, not the flu
Just like in 1918, our problem in the coming pandemic may be that we have antibiotic agents but they are not effective against the infections that will result.
Once again, the most effective panacea may be isolation and that means a Vault somewhere with proper decontamination and seals against carriers from outside.