Make no mistake: The challenge is huge and unprecedented. If a typical virus is a riddle wrapped in a mystery inside an enigma, the coronavirus known as SARS-CoV-2 is all that and more. It combines contagiousness and lethality in a ferocious mixture that Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, has called his "worst nightmare." For one thing, when it appeared, no one on Earth had immunity. Second, it's airborne and infects the upper respiratory tract, which means it's readily spewed back into the air where it can drift from person to person. Third, and arguably worst, the virus is most contagious before it causes symptoms, meaning that carriers feel well enough to be up and about precisely when they're most likely to infect us.

The tricks this virus uses to foil the body's counterattack are diabolically effective. Once it gets inside via the nose or mouth, the coronavirus eludes the first line of immune defense, slides easily into cells, churns out copies of itself by hijacking the cell's machinery, and makes sure those copies work by using a proofreading mechanism that many other viruses don't even have. Its effect is relentless: It can turn a person's lung cells into useless material that looks like ground glass; blow open blood vessels or destroy them with microscopic clots; and gunk up the workings of a kidney, heart, or liver, rendering them too stiff to repair. It can defang the cells that attack invading viruses and then provoke a secondary immune reaction that goes badly haywire, paradoxically causing its own catastrophe. And anyone who comes in close contact with a person already infected is likely -- though no one is sure how likely -- to get infected too.