Are face masks going to become like condoms — ubiquitous, sometimes fashionable, promoted with public service announcements? They should be, one virus researcher says, if early indications are correct in suggesting that Covid-19 is often spread by people who feel healthy and show no symptoms.

David O’Connor, who studies viral disease at the University of Wisconsin-Madison, said: “If a substantial amount of transmission occurs before people feel sick, how do you stop that? By the time people feel sick and seek care, all the testing and isolation in the world would be too little, too late.”

Dr. O’Connor, who researches H.I.V. and other viruses, including the new coronavirus, said some recent research had shifted his thinking about the current pandemic.

“H.I.V. is also spread while people feel fine,” he wrote in an email, “and consistent, correct condom use is a barrier to sexual virus transmission that works.”