Please excuse my lack of knowledge on this subject Eric but in your opinion why is it disease outbreaks almost always start in Africa, AIDS, Ebola etc?
Tropical areas tend to have more biodiversity.
Add warm weather and humidity and pathogens that would have viable only a few seconds in most parts of the US survive minutes or even hours outside of host.
This makes a huge difference in the ability of the pathogen to spread.
Even more important 1st world hygiene is much more established than in the 3rd world.
The combination of those factors plays the biggest role.
Another location to watch for outside of Africa is China.
The proximity of larger populations of birds with pigs and humans makes for a stew that favors mutations.
IMHO a more lethal form of the Influenza is the greastes pandemic threat because unlike Ebola it transmits airborne.
( I dont want to write too much on this but aerosol transmission can exist for any pathogen as it did for Ebola in some historical examples.. this does not change the distinction tho that Ebola is not airborne which is a specific ability of a pathogen to remain viable while dried out so is able to float longer distances as tiny(er) particles in air.. this Ebola cannot do, but Flu cna)