Flu viruses get less dangerous over time. The more deadly strains literally kill off their hosts, making those strains less likely to spread.
It it experience based on historical data. A virus jumping to a new host (species) will be more aggressive in the beginning until it has adopted (mutated) to his new host.
While this is not a universal truth, it is experience and could also be backed up mathematically (>host dies to soon, virus can not spread) and by game theory.