So the probability is that in 1979, small pox wouldn't really become an epidemic...
I tell you what, don't say small pox, say a specially weaponised version of Ebola... Call it Ebola-C or something. Kills rapidly and is very easy to pass on. Waterborne lets say. They were ambushed by a river.
Yes it's coincidental, but only as much as some books.
Someones been reading Tom Clancy