In February, as the new coronavirus swept across China and shut down entire cities, a scientist named Sai Li set out to paint its portrait.

At the time, the best pictures anyone had managed to take were low-resolution images, in which the virus looked like a barely discernible smudge.

Dr. Li, a structural biologist at Tsinghua University in Beijing, joined forces with virologists who were rearing the virus in a biosafety lab in the city of Hangzhou. Those researchers doused the viruses with chemicals to render them harmless and then sent them to Dr. Li.

Dr. Li and his colleagues then concentrated the virus-laden fluid from a quart down to a single drop. He could only hope that they had done everything just right, so that the weeks of work to produce that drop would not have been a waste...[+]