Bruce L. Davidson MD, MPH is a pulmonary physician and researcher in Seattle, an expert in respiratory transmission of infection, and former president of the National Tuberculosis Controllers Association and member of the Advisory Council for the Elimination of Tuberculosis. The opinions expressed in this commentary are his own. View more opinion on CNN.

(CNN) What you may not know about coronavirus -- the disease that has reached pandemic proportions -- is that one of the times you are most vulnerable to contracting it is while you're sleeping. Here is how this can happen, and what you can do about it.

Coronavirus infects cells below the voice box, in the airways and deep in the lungs , unlike flu viruses which start with your nose and throat. Other than via tiny particles inhaled in air, coronavirus reaches those cells via fluid in the nose or throat that sneaks past your voice box (this is called aspiration) and slides down your windpipe, or trachea.

If you have lung damage from smoking, swallowing saliva, which we all do while we sleep, can deliver even more aspirated throat contents into the lungs. The amount of fluid aspirated during sleep is enough to cause pneumonia -- in fact, it's how most pneumonia occurs.

So why don't we wake up with pneumonia every morning? First, most of our noses and throats carry safe bacteria, not microbes like coronavirus that can cause pneumonia . Second, our deep lung has some cells that kill microbes that arrive there.

Lastly, healthy people have a robust system for transporting the aspirated viruses and bacteria up the windpipes and dropping it in the esophagus, where it is swallowed and begins to be digested with the saliva we make. Some people get mild diarrhea from this virus because it can infect cells in your gut, too.

Daytime (waking) aspiration, such as when food goes down the wrong pipe, is coughed up -- if you have a normal cough reflex and a strong cough. Each of these protective factors for relatively young and healthy people can be impaired with lung disease and advancing age.

Getting sick from coronavirus likely depends on the relative weights of two factors: vulnerability to lung infection, and how many coronaviruses get into the lung. A vulnerable person, with lungs or airway function impaired by years of smoking, or persistent asthma, or immune compromise from drugs or disease, is more at risk from even a small number of viruses.

Healthy, non-vulnerable people are more likely to resist and "clear" a small number of viruses, or maybe just get a little sick, after chance contact from an infected person -- but they would still be susceptible if they are exposed to a higher amount.

How can you limit your personal exposure to coronavirus? Move away from people breathing in your face, avoid indoor close gatherings of people, and stand some feet away even when outside. Dress warmly so you can seek blowing air, even when it's cold. Don't spend long in a poorly ventilated rest room where someone may have coughed hard moments ago.

Be extra careful if you are a vulnerable person. Sufficient masks must be made readily available for hospitals, clinics and other caregivers. Everyone else, meanwhile, should use avoidance to minimize virus particles deposited near your face.

And since you may well have been exposed anyhow, remove any coronavirus already around your nose and throat: Wash your hands and face well with soap and warm water, including -- on a finger -- a quarter-inch into each nostril. Then gently blow your nose. DON'T use those irrigating devices, like neti pots, that might force virus further inside! Brush your teeth and tongue, swish and spit, and gargle once or twice with an antiseptic mouthwash.

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The key is to minimize the virus burden around and inside your face before you go to sleep.

Limit sedation before sleep during an epidemic. If you use a device at night for sleep apnea, make sure it remains away from where people could cough, sneeze and breathe on it, and clean it regularly.

That's how to minimize exposing your lungs to any lurking coronavirus dosage you might encounter.