Myths, half-truths and wives’ tales persist in medicine. Sometimes doctors and nurses believe things that aren’t true or at least are unproven. That’s the focus of a new book, “Don’t Swallow Your Gum! Myths, Half-Truths and Outright Lies About Your Body and Health,” by Dr. Aaron E. Carroll and Dr. Rachel C. Vreeman.

Dr. Carroll and Dr. Vreeman, both from the Indiana University School of Medicine, have written a lot about medical myths and misunderstandings. In December 2007, the pair published an article in the British Medical Journal about seven medical myths even doctors believe. A year later, they identified six medical myths for the holiday season.

Now, “Don’t Swallow Your Gum” offers a fun collection of numerous medical myths that are likely familiar to most of us. The book is organized to focus on various types of myths, including myths about your body (you should poop at least once a day), myths about disease and illness (cold weather makes you sick), myths about sex and pregnancy (twins skip a generation) and myths about what we eat and drink (gum stays in your stomach for seven years.)

It’s a fun read, and chances are you will stumble across several medical myths you’ve always believed. Here are a few medical myths that may surprise you:

1. Cold weather makes you sick. In studies of cold transmission, people who are chilled are no more likely to get sick than those who were not. It may be that cold weather keeps people indoors, where germs are more likely to catch up with you.

2. Green mucus indicates a sinus infection. The importance of mucus color is a medical myth even doctors believe, the authors say. “There is no evidence…that antibiotics shorten the duration of an illness when green snot is a symptom,” they write.

3. You lose most of your body heat through your head. There is nothing special about the head and heat loss. You will lose heat through any uncovered body part.

4. Milk makes you phlegmy. In a study of 330 patients, nearly two out of three believed milk increases phlegm production. But it’s not true. In one experiment, volunteers were infected with the cold virus, and some of them drank a lot of milk as well. The weight of the nasal secretions did not increase in those who drank more milk, nor was it associated with cough or congestion.

5. Cracking your knuckles will cause arthritis. Knuckle-crackers are no more likely to have arthritis than those who don’t make annoying popping sounds with their fingers.

6. Birth control pills don’t work as well with antibiotics. A review of the literature concluded that common antibiotics don’t affect birth control pills. “It is much more important to take your birth control pill every day at the same time than to spend time worrying about your antibiotics,” the authors write.

7. Singles have better sex lives than married people. You may think your bachelor friends are having all the fun, but single people also go through a lot of dry spells when they aren’t dating anyone. The result — married people typically have more sex in a given year than single people. In one survey, 43 percent of married men reported having sex two to three times per week, compared to only 26 percent of single men. The numbers were slightly lower but similar for women. Married people are also more likely to have orgasms and give and receive oral sex.

8. Sugar makes kids hyper. Numerous studies show sugar doesn’t affect behavior, but most parents don’t believe this. In one study, parents were told their kids had sugar and they were more likely to report problem behavior — but in reality, the kids had consumed a sugar-free drink.

9. You should poop at least once a day. A half-truth, say the authors. Regular bowel movements prevent discomfort and constipation, but a perfectly healthy person may not move their bowels every day. Constipation is defined as having fewer than three stools per week.

10. It’s okay to double dip in the chip dip. In one study, scientists took a bite of cracker and then dipped it into salsa, cheese dip, chocolate syrup and water. They did the same test with a fresh, unbitten cracker. Then they measured bacteria in the dips and the volunteers’ mouths. On average, three to six double dips transferred about 10,000 bacteria from the eater’s mouth to the dip. And each cracker picked up between one and two grams of dip. Salsa picked up the most germs from double dipping.

11. Food quickly picked up from the floor is safe to eat. Scientists have put the commonly-cited five-second rule to the test. They found that food that comes into contact with a tile or wood floor does pick up large amounts of bacteria. Food doesn’t pick up many germs when it hits carpet, but it does pick up carpet fuzz.