In 2007, Dr. Denison discovered that coronaviruses have a powerful “proofreading” system. If an error occurs in copying RNA as the coronavirus replicates, it corrects the error. In lab experiments, coronaviruses that mutated were weaker, outcompeted by those without mutations.

Dr. Denison and other experts wondered if it might be possible to trick the virus with a drug that dodged the proofreading system and blocked the virus’s growing RNA chain, making it prematurely terminate.

Talking about this problem with another scientist at a meeting, Dr. Denison learned that Gilead Sciences had dozens of drugs that might do the trick. “All of these compounds had been shelved for one reason or another,” Dr. Denison said.

Most worked in lab tests to shut down coronaviruses, he found — some better than others. One of the best was GS-5734, now known as remdesivir. “I like to call it the Terminator,” Dr. Denison said.