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Viral particles travel via airborne droplets of saliva, mucus and cells that make their way into a new body when their unsuspecting host breathes them in.

Doctors were forced to intervene to clear mucus from the Pope's lungs, an update from the Holy See said, though it said he was alert throughout.

Cystic fibrosis is an inherited, life-threatening disorder that affects multiple organs and is caused by a defective gene that leads to the production of thickened mucus, which clogs airways and the digestive tract.

Pregnancy itself makes people more physically vulnerable, with changes to breathing patterns, heart function and mucus membranes that can intensify the effects of smoke and air pollution.

Beyond obvious symptoms such as a scratchy throat and runny nose, Hur urges people to watch for changes in their sense of smell or whether their normally thin, clear mucus becomes thicker.