Efthimiou assumed that the first vampire came into existence on January 1st, 1600, at which point the human population was 536,870,911. Supposing the vampire fed once a month and each victim transformed into a vampire, there would be two vampires and 536,870,910 humans on February 1st of the same year. Following this, four vampires would exist on March 1st and eight on April 1st. If this pattern continued, all of the original humans would become vampires within two and a half years, effectively devastating the vampires' food source. - See more at: http://1degreebio.org/blog/?bid=547#sthash.SczXpWvZ.dpuf