Quiz answer:  When cold rainy 35 degree 50% RH outside  air is sucked.in
either by  mechanical ventilation  or  infiltration and  heated  to 70 degrees  the
resulting RH of that air is 11%.

Viruses attach to dust particles which can remain suspended in air with  RH
below 35%. We, of course, breath in the dust particles and the viruses. Dust
particles in air above 40% RH absorb enough  moisture to drop  to the floor
along with the viruses.  Now you know the rest of the story.
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                                   Stay healthy, get humidified !