Dormant genes

This alien race had planned its invasion of earth for a long time. For dozens of years, they had secretly injected some of their DNA in unsuspecting humans, during their sleep. Those humans were living their lives, going to work, raising their family, without thinking that they were part of a devilish plan to take over the world, and that it would only take one trigger to activate their dormant genes and turn them into twisted, blood-thirsty creatures with nothing but hatred towards the human race.