the trick for a virus to go worldwide is the length of the contagious phrase and the level of infectiousness.
if it kills the host too quickly, then it has a lower probability to spread.
with the advent of airlines, the possibility of world wide contagion has dramatically increased. prior to airlines, the 30 days of travel would wipe out an entire ship... but would not make it to its destination.
airlines resolve this and magnify it... by keeping everyone in a single tube, in close quarters, breathing the same air (assuming air-born virus)... and the host doesn’t die before reaching the destination... thereby spreading the virus geometrically.
The movie "Twelve Monkeys" with Bruce Willis and Brad Pitt is a strange movie and not to everyone's taste (but I like it). There is a bit in there about how airports can spread disease very effectively. I won't say more than that.