I have had two math courses when I was attending Syracuse University as a full time student near fifty years ago "finite mathmatics and probability theory" and "a brief introduction to elementary calculus" which I got an A and B+ in respectively which explain much of this mathmatical start and cessation of flu influenza. With Avian H1N1 hp lethal virus high pathogenic of 1918-19 we are talking massive infection by AH1N1 leached out of the freeze flu viral strands deposited by geese, ducks, gulls as far back as the Mastondons. These were just ordinary run of mill H1N1's during Mastondon times. However now so many dozens of centuries later people have lost immunity to many of them. However the majority retain enough immunity to counter. A massive infection that occurred at Spitsbergen 1918-19 to ducks, geese, gulls lead to a bringing down of massive infection on the globe following nesting season. Spitsbergen birds leave early for the south so this meant massive infection beginning by the last week in August 1918 which quickly spread around the world massively because of the beginning massivity of infection at Spitsbergen from the ancient leaching virus because of the Spitsbergen freeze warmed to red line by summer 1918 by the exploding WW1 naval ammunitions to the south. The waters within two years had cooled down however ending this leaching condition and the source. The viral LOAD is also important. Relatively high LOADS lead to infection and relatively lower LOADS lead to a building up of immunity. Thus with the lethal infefctions immunities were also built up that in a couple of years by virtue of built up immunity put an end to lethal Avian H1N1. And new leached out strands were not being brought down by nesting geese, ducks and gulls as the waters causing leaching had cooled. It is a well known fact that influenzas can spread from one origin around the globe in a half year. But the more reduced LOADS which also accompany high LOADS create immunity. Congestion fissioning is also an important variable. There are many variables. An example would be of duck and geese ponds around mexico city generating strands of H1N1 relatively old virus in the cool 10,000 foot altitudes of mexico city (I have been snowed on in Mexico City in August) but these strands will not be nearly as old as those in the Spitsbergen freeze and still people will not have lost as much immunity to them as those going back to the Mastondons in the Spitsbergen freeze where atop a mountain the north polar ice cap can be viewed. Yet these Mexico city strands are infectious enouth to become a severe influenza problem when fissioned in the Mexico City subway. They will probably remain local in terms of the fissioning in the Mexico City subway which made this strain noticable in the first place. It is basically an average strain. The strain is still an average flu and it does not kill many. And in fact only a few in its epicenter Mexico Ctiy. Thus if it reaches Bejing by a mexican traveler and he or she infects somebody nobody notices. It is an average AH1N1 virus and does not infect anyone else in Bejing.