NEW YORK — A delicate but highly contagious virus, around 900 times smaller than the width of a human hair, is spreading from person to person around the world. The coronavirus, as it’s known, has already infected people in at least 60 countries.

Because this virus is so new, experts’ understanding of how it spreads is limited. They can, however, offer some guidance about how it does — and does not — seem to be transmitted.