In 2015, a Sydney university observed medical students on video and recorded how many times they touched their faces. Each of the 26 future doctors under observation touched their faces an average 23 times per hour.

Nearly half of those times -- 44% -- involved contact with their eyes, nose or mouth.

Not touching your face is harder than it sounds

As we fight the coronavirus outbreak, which has killed 19 people in the US and more than 3,500 worldwide, officials are emphasizing all it takes is one touch for microbes on your fingers to slip into your body through your nostril, eyes or mouth.

"Viruses that affect the respiratory system enter the body through mucosal membranes which are found in the nose, oral cavity and lips. With poor hand hygiene, it's easy to acquire a viral infection this way," says Dr. Dawn Mueni Becker, an infectious disease specialist in Gainesville, Florida.

But we've been touching our faces all our lives, and stopping that habit is easier said than done.

On Friday, a video widely shared on social media showed a California health official touching her face during a news conference advising people not to do that as the coronavirus outbreak spreads.

"Start working on not touching your face, because one main way viruses spread is when you touch your own mouth, nose or eyes," she says. Then she licks her finger to flip to the next page of her remarks -- seemingly unaware she was not following her own advice.

Even one notorious self-proclaimed germaphobe can't help himself. "I haven't touched my face in weeks -- in weeks. I miss it," President Donald Trump said jokingly last week. He was photographed touching his face Monday.

Touching your face is at times tied to stress

Like most behaviors, constant face touching starts at a young age and becomes a habit over time. People touch their face for various reasons. One 2014 federal government study suggested it helps reduce stress and discomfort

"Spontaneous facial self-touch gestures are performed manifold every day by every human being, primarily in stressful situations," the study says. "These movements are not usually designed to communicate and are frequently accomplished with little or no awareness."

Touching your face is so common, there's a website that uses your webcam to notify you when you touch your face and keeps track of how many times you do it.

There are safe ways to touch your face

If you can't stop yourself, it's not the end of the world, Becker says. There are ways to lower your chances of infections from face touching.

"Being conscious or aware of this habit is helpful when it comes to avoiding touching the face," Becker says. "Identifying triggers such as runny nose or urge to sneeze is important. In this case, having tissue close by is helpful -- it's better to use that to touch your face than bare hands."

The federal Centers for Disease Control and Prevention recommends washing your hands with soap and water for at least 20 seconds. If water is not available, you can use an alcohol-based hand sanitizer that contains at least 60% alcohol.

Soap and water are especially preferred if hands are visibly dirty, the CDC says, and especially after using the bathroom, blowing your nose, coughing or sneezing.

Think of all the germy things people touch all day. Cellphones, car keys, doors and elevators -- even wads of cash that you have no idea the places they've been. Now imagine transferring all the bacteria, viruses and allergens from those items into your body through the mucous membranes in your nose, mouth and eyes. Or through a cut in your face or neck that you have no idea exists.