"So here is the more complete CDC advice: "Wear a mask, stay at least 6 feet apart, avoid crowds, and wash your hands often.""

It is unbelievable that this list of behaviors does not include airing out or ventilating indoor public spaces.

Here in sunny California, where the weather in the Bay Area has been mild and warm through December 3, I find myself walking into shops or offices every day that have operable windows and doors: all shut tight.

Many of these spaces could be indistinguishable from the outdoors in terms of fresh air flow. For reasons I cannot fathom this simplest (and healthiest) mitigation is not practiced.