In a famous mid-20th-century study in Salisbury, England, a group of volunteers walked through cold corridors in wet socks and bathing suits"for half an hour or as long as they could bear it" every day for two weeks, while a second group was kept warm and dry. Researchers dripped infected mucus into the nostrils of volunteers in both groups, Norris told the Penn students. After two weeks, volunteers in both groups caught colds at the same rate.

Later studies have found similar results, indicating that cold weather is not directly to blame for transmitting cold viruses. And yet, people are indeed more likely to catch colds when the weather is cold. Physicians suspect this is partly because we spend more time indoors exposed to other people's germs. Dry air also may play a role -- perhaps because dry mucous membranes are less effective at warding off germs, or because viral particles stay aloft longer in dry air.

And here’s another myth: that herbal remedies can stop a cold. Generally, the evidence for echinacea and other such "treatments" has not held up to the scrutiny of science, Norris said. Nor can pharmaceuticals, so stop asking your doctor for antibiotics you don't really need.

The vast majority of colds are the result of infection with a type of germ called a rhinovirus, which makes sense because the prefix rhino- comes from an ancient Greek word for nose. The lips, tongue, and saliva, on the other hand, are relatively barren in terms of harboring rhinoviruses, Norris said.

So colds are very hard to spread by sharing a drinking glass. Someone would have to rub his drippy nose, then pick up the drinking glass, then share it with someone else while the nasal secretions were still fresh, Norris said. Then that second person would have to touch his or her own nostrils after using the glass.

Same goes for kissing, which presents little risk of cold virus transmission.

"Unless your kissing technique involves using your nose," Norris joked.

Conventional wisdom is on target when it comes to stress and lack of sleep, Norris told the Penn medical students, citing the results of controlled studies in which researchers purposely exposed volunteers to cold viruses.

A higher rate of psychological stress is associated with a greater likelihood of catching a cold. Researchers also have found that people who sleep better are less likely to get sick. This has been shown both in studies in which participants self-reported their nightly duration of sleep, and in studies in which sleep duration was measured with an actigraph – a wrist-mounted activity sensor.

In one study, people who averaged less than seven hours of sleep a night were three times as likely to catch colds as those who averaged more than eight hours a night, Norris said.

As to why stress and sleep deprivation raise the risk of colds, one might suspect the reason is that the body’s defenses are worn down. In fact, Norris said, the true connection is unclear.

There is a $2 billion industry devoted to toilet-seat hygiene, with merchants offering all manner of sanitary wipes, seat sanitizers, and protective covers.

Save your money, Norris told her class of chuckling med students. Transmission of germs occurs through the mucous membranes, such as those in the nostrils. The skin on your rear end is "highly keratinized" (keratin is the fibrous protein in skin) and is a good barrier against disease, she said. The only way a sexually transmitted disease might get spread by a toilet seat is if you have an open sore or if your hand touches a dirty surface and then touches a mucous membrane, such as your nostrils, eyes, or genitals. Which is why you always want to wash your hands after using the toilet. Hand sanitizers may help, but the tried-and-true approach is a thorough hand-washing with soap and water.

And think twice before texting or checking your phone inside that stall. In one study, 75 percent of people admitted texting while on the toilet, but Norris said that likely increases the chance of microbial transmission.

"Do you wash your phone?" she asked her students. "No."

Wait until after washing your hands to check your messages.

With sophisticated ventilation systems and air filters now common in airplanes, the risk is much lower than it was decades ago.

The rule of thumb is that infectious diseases are unlikely to spread through the air unless the flight lasts more than eight hours, and even then the risk is largely limited to those within two rows of an infected passenger.

An exception to this rule is measles, which is highly contagious. The best defense in that case is widespread vaccination, Norris said.

It depends on what is meant by "cleaner." There are plenty of bacteria in a dog’s mouth, but they are different from what is found in a human’s mouth and most are not known to cause human disease.

Dogs are known to lick their wounds, giving rise to the belief that canine saliva has some sort of therapeutic powers, Norris said. In reality, any salutary effect from licking is likely due to "debridement" – the removal of any dead or damaged tissue.

As for that belief that a human bite is more dangerous from an infection standpoint than a dog’s, the truth is unclear. Though even a mild dog bite often prompts a trip to a doctor, evidence suggests that only the most severe human bite wounds receive medical attention, perhaps giving the impression that they are riskier than they really are, Norris said.

So is a dog’s mouth cleaner than a human’s? It depends on the hygiene of the human, and on the age and health of the dog, Norris said.

Soaking a toothbrush in disinfectant may lead to cross-contamination if the solution is used by multiple users, according to a fact sheet Norris cited from the U.S. Centers for Disease Control and Prevention, so do not bother. It also may be unwise to try to disinfect a toothbrush with dishwashers, microwaves, or ultraviolet devices, as the toothbrush may become damaged, the CDC says.

Instead, rinse the toothbrush thoroughly after each use to remove debris and toothpaste, and store it upright in a ventilated space so it does not touch other toothbrushes. And never share a toothbrush, because even after rinsing it might harbor germs that could hurt someone else.