The Hallmark version of Thanksgiving involves friends and family gathered around a beautifully set table for a delicious holiday meal, maybe sharing a bite of pie.

In real life, it might also involve that cousin who always double-dips. Or, in a worst-case scenario, scooping that beautifully roasted turkey up off the floor after you dropped it on the way to the dinner table.

Just how worried should we be about these prospects?

Food-borne illnesses cause an estimated 48 million illnesses and 3,000 deaths each year in the United States, according to the U.S. Centers for Disease Control and Prevention. A new book challenges some pervasive food myths and takes a tough look at the safety, or lack thereof, of some common behaviors.

Paul Dawson is a professor of food safety at Clemson University and co-author, with Brian Sheldon, of Did You Just Eat That?: Two Scientists Explore Double-Dipping, the Five-second Rule, and Other Food Myths in the Lab.

So where does the five-second rule come from, anyway?

Dawson says no one knows for sure, but Genghis Khan has been quoted as saying that anything dropped on the floor was good enough to eat because it was prepared for him. Julia Child once dropped a potato pancake on the stove top during her cooking show and announced that it was okay to serve it as long as nobody was in the kitchen.

But the truth is, Dawson said, if there are pathogens on the surface that the food touches, it doesn’t matter how many seconds the food lies there. He tested it in the lab.

“There was plenty of bacteria there in five seconds,” Dawson said, though there was more bacteria after 30 seconds.

Make sure the surface that you drop the food on is clean, Dawson advised. If you just prepared raw chicken there, it’s not safe.

What about double-dipping, sharing food and blowing out birthday candles?

There’s not a huge concern for infecting yourself with salmonella or Campylobacter, Dawson said, because most people don’t carry those bacteria in their mouths. However, people do carry cold and flu viruses in their saliva.

“If you have an elderly relative there who may be have a reduced immune system, you may not want them eating the cake that the 5-year-olds were blowing out,” Dawson said.