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The holidays are upon us, and with them all sorts of celebrations that involve food and drinks. Many of us will engage in typical eating-related behaviors — such as looking at a menu, sharing a bite with a friend or double-dipping that carrot stick — without a second thought. Unfortunately, these seemingly innocuous habits can spread disease, as we learned through the many germ-laden experiments we conducted for our new book, “Did You Just Eat That?”

No matter the occasion, humans can encounter or transmit infectious agents in a variety of ways — by touching surfaces or people, sharing food or drinks, or coughing or sneezing. The microbes involved can be related to foodborne illnesses, such as E. coli or salmonella, or can be viral disease agents that cause the common cold and influenza. Contagious disease agents often lurk in the bioaerosols — tiny water droplets — coming from our mouth and nose. These droplets are about 10- to 100-plus-times larger than bacterial cells and viral particles, providing plenty of room for nasty microbes to ride along when the bioaerosols are expelled during breathing, coughing, sneezing or even blowing out birthday ­candles.

Other than wearing gloves and masks wherever we venture, what’s a partygoer to do? If you want to stay healthy, here are six situations to avoid this holiday season.

Eating birthday cake

Will any of your holiday celebrations coincide with someone’s birthday? If so, you might want to politely decline a slice of cake: Our studies showed that blowing out birthday candles can transfer oral bacteria to the cake surface. In our experiments, nearly 3,000 more bacteria — and as many as 37,000 bacteria — were recovered from the surface of birthday cakes after the candles were blown out, compared with control cakes where candles remained lit.

Using hand air dryers

If you’re in a public restroom, try to avoid the hand air dryers, which blow bacteria around along with the hot air (it is for this reason that they are not recommended for medical facilities by several organizations, including the Infectious Diseases Society of America). In our study, we found an average of more than 18,000 bacteria on restroom hand air dryers in grocery stores, and more than 2,000 in gas stations and on a college campus. (In general, the activation push buttons and air intake vents in male bathroom dryers had more bacteria than hand dryers found in female bathrooms.) One study out of the United Kingdom found that hand air dryers increased bacteria populations on hands fivefold after washing, while paper towels decreased bacterial populations on hands by 42 percent. In our estimation, hand towels are your best option.

Requesting ice and lemons

In developed countries, it’s rare for ice to be contaminated before freezing, but it does happen: In 1987, ice-borne Norwalk virus sickened more than 5,000 people in Pennsylvania, Delaware and New Jersey. What’s more likely is that bacteria and viruses will be transferred from hands touching your ice and lemon before they are plopped into your drink. We inoculated our test subjects’ hands with E. coli bacteria and then had them touch wet lemons or cubes of ice. The result? More than 6,000 E. coli bacteria were transferred to 100 percent of the wet lemons or ice cubes touched by test subjects’ hands.

Sharing food

Psychologists have found that sharing food increases intimacy, but you are also sharing your oral microbes. We found that 70,000 more bacteria per milliliter were transferred to a bowl of broth from a spoon placed in the mouth than from a spoon not placed in the mouth. The transfer population to rice via the mouth/spoon route was 800,000 bacteria per gram of rice; more than 2 million bacteria per gram were transferred when using hands alone (a common practice in some cultures).

And before you head out to the cinema for that holiday blockbuster, consider: Eating popcorn in a movie theater may also have hidden risks. First, the seats and armrests you touch just before diving into a bag of popcorn have been touched by many other people and are probably contaminated. A report by the ABC show “20/20” found that the seats and armrests in New York and Los Angeles movie theaters contained fecal bacteria. If you touch a seat or armrest and then dive into a bucket of popcorn, such germs can contaminate your snack. Our testing showed that 85 percent of the handfuls of popcorn touched with hands inoculated with E. coli contained these bacteria, while 79 percent of the popcorn samples remaining in the serving bowl contained E. coli transferred from the person taking a handful of popcorn from the same bowl.

Double-dipping

George Costanza of “Seinfeld” was wrong and Timmy was right! Double-dipping is like putting your whole mouth right in the dip. We found that on average, between 100 and 1,000 bacteria were transferred from the mouth to the dip by double-dipping a cracker. And, as you might expect, the thinner (less viscous) the dip (salsa vs. cheese or chocolate), the more bacteria transferred from the mouth to the dip, because a greater volume of the thinner dip drops from the cracker back into the bowl. On the other hand, after two hours at room temperature, the number of bacteria in the salsa had dropped to similar levels as those in the chocolate and cheese dips because of its acidity level, but the salsa still had significant levels of bacteria.

Handling a menu

When you sit down to order a meal, bacteria and other microorganisms are on nearly every surface around you. The surfaces to be really concerned about, however, are menus, which are touched numerous times by restaurant staffers and customers with unknown personal hygiene habits. In our study, we detected more than 2,000 bacteria on six-inch- square samplings of 108 randomly sampled restaurant menus. In another study, we found from 1 to 32 percent of the E. coli bacteria that we had inoculated on menus were transferred to our participants’ hands after they had handled the menus for one minute.

There are more examples in our book (for example, the five-second rule is bunk!). But as this summary of several of our food-related experiments makes clear, everyday activities we unthinkingly engage in can pose serious hazards to our health. The degree of danger depends on many factors, including the type and population of disease-causing microbial pathogens present, the level of exposure to these pathogens (how many make it into our bodies), our general health status (the elderly and very young or immunocompromised are more at risk), and the general health status and level of infection of those who might be contributing to the contamination of a surface, our food, the air around us or directly to us via personal contact.

Besides avoiding these six situations when possible, one of the best ways to stay healthy is to practice good personal hygiene habits and encourage them in those around you. These would include effective hand-washing (10-second warm water rinse, ­10- to 15-seconds lathering and scrubbing of the hands with soap, warm water rinse long enough to remove all of the lather, and then drying with disposable paper towels); covering your mouth when coughing and sneezing; keeping your hands out of your eyes, nose and mouth; and staying home when you are sick and limiting your contact with those who are sick. Do that, and hopefully your holiday season will be memorable for happy gatherings without any unpleasant fallout.

Paul L. Dawson is a professor in the Food, Nutrition and Packaging Sciences department at Clemson University. Brian Sheldon is professor emeritus in food microbiology at North Carolina State University. Their book, “Did You Just Eat That? Two Scientists Explore Double-Dipping, the Five-Second Rule, and Other Food Myths in the Lab,” was published last month by W.W. Norton.