How does Infection Bio War calculate infection spreading (infecting)?

I was playing Infection Bio War, but it certainly has a different way of calculating how the infection expands than other games in the genre. When you open the stats tab while all states are infected, the game shows you like 10x more states with over 90% or under 10% infected people then the other states.

How do the infections spread from across countries?
How does this differ from how the infection spreads inside a state?