You’re more likely to catch a cold if you don’t wear a hat or scarf. You need to drink eight glasses of water a day for proper hydration. If your mucus is green, you need an antibiotic.

You've heard some of this stuff for so long, how could it not be true?

Easy, says Rachel Vreeman, MD, director of research for the Indiana University Center for Global Health and author of health myth books, including "Don't Swallow Your Gum." "These myths stick around even after they have been debunked," she says. "They seem to have a life of their own."

Related: Quiz: Flu Myths and Facts

They are hard to dismiss for several reasons, she says. Many of them come from people we trust, such as parents or teachers. Some seem to help explain common conditions — and if that explanation makes sense, we tend to accept it. And, in some cases, the myth may be generally true, but with some exceptions.

Which of these have you heard?

Myth: Sugar makes children hyper

kid with candyResearchers pooled the results of several studies, Vreeman says, and concluded that sugar doesn’t wind up the little ones.

Some parents expect to see their kids become more hyperactive after eating sugar and therefore may rate their behavior as rowdier when researchers question them, Vreeman says. But in reality it has no impact, at least on that aspect of behavior.

Still, ''there are plenty of reasons health wise not to give kids sugar," Vreeman says.  (Photo: gosphotodesign/Shutterstock)

Myth: If your mucus is green, you have a bacterial infection and need antibiotics

Doctors love to ask the color of your mucus, Vreeman says. And "a lot of doctors believe the color of the mucus matters." But it doesn’t. "It could be green and you could not have an infection."

Related: Do You Really Need That Antibiotic?

Myth: If you drop food on the floor and pick it up within 5 seconds, it’s fine to eat

"If there is bacteria on the floor, which there almost always is, it will transfer to the food surface," says Vreeman.

It's a matter of degree, however, and the flooring surface matters, according to British researchers. They found that bacteria was most likely to transfer from tile to food after the food was there for more than a few seconds — but that infection risk was there from the get-go. Surprisingly, carpet is the best surface for food to land on; the researchers found it’s the most hostile to bacteria. Tile was the best breeding ground for bacteria, followed by wood floors.

Of course, whether or not you get sick from that bug-laden cookie or cheese stick depends on a number of factors, Vreeman says, including how healthy you are at the moment and how many bacteria you ingest and what type.

Myth: If you’re on birth control pills and need an antibiotic, you’d better use a second form of birth control

"This is a longstanding idea, that birth control pills don't work as well if you are on antibiotics," Vreeman says. Not true, in general, she says. But there are exceptions. According to the American College of Obstetricians and Gynecologists, two antibiotics, rifampin (Rifampin) and griseofulvin (Grifulvin V), may affect the way the pill works.

deodorant womanMyth: Antiperspirant increases your breast cancer risk

This myth, which was once widely circulated on social media and via email, claimed that substances in antiperspirants seep through nicks from underarm shaving. When they reach the lymph nodes, the idea went, they get stuck there and produce toxins, and cancer eventually results.

The American Cancer Society debunks this one. One study compared women with and without breast cancer and found no link between antiperspirants and breast cancer or underarm shaving. (Photo: Alliance/Shutterstock)

Myth: Everyone needs 8 glasses of water a day to stay hydrated

Actually, the amount of water you need varies. It depends on various factors, including your general health, how much activity you engage in and the climate where you live.

According to the Institute of Medicine, an adequate daily intake of fluids (that includes plain water as well as your morning cup of coffee, tea or orange juice) is about 13 cups for a man and 9 cups for a woman. 

Related: How to Stay Hydrated Without Gulping Water

A “myth” that may or may not be true

woman cold in winterIf you get chilled when you’re outside, perhaps because you failed to bundle up sufficiently, you're more likely to catch a cold, or so the thinking goes.

"This is really well studied," Vreeman says, and the verdict, he says, is clear: "You are not any more likely to get sick. It's not the cold weather, it's the viruses that give you a cold."

That said, Yale researchers recently discovered that the cold virus does replicate better at cooler temperatures thanks to a dampened immune response. Their research was done on cells taken from the airways of mice, though, it’s not yet clear how, or whether, this applies to humans under real-life conditions.

According to study senior author Akiko Iwasaki, PhD, ”the research may give credence to the old wives’ tale that people should keep warm, and even cover their noses, to avoid catching colds.”

Stay tuned on this one. (Photo: Andresr/Shutterstock)

Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.