This is a big food mystery. Apparently the internet has been buzzing for a while about the so called five second rule which is a time limit of how long food has to be on the ground before it becomes unsafe to pickup and eat. Here is some research on the myth from a lab at Clemson University:
So is five seconds on the floor the critical threshold that separates an edible morsel from a case of food poisoning? It’s a bit a more complicated than that. It depends on just how much bacteria can make it from floor to food in a few seconds and just how dirty the floor is.
Let’s find out the results of their study on page 2