This article has been updated. A few sentences have shaken a century of science. A week ago, more than a year after the World Health Organization declared that we face a pandemic, a page on its website titled “Coronavirus Disease (Covid-19): How Is It Transmitted?” got a seemingly small update. The agency’s response to that question had been that “current evidence suggests that the main way the virus spreads is by respiratory droplets” — which are expelled from the mouth and quickly fall to the ground — “among people who are in close contact with each other.” The revised response still emphasizes transmission in close contact but now says it may be via aerosols — smaller respiratory particles that can float — as well as droplets. It also adds a reason the virus can also be transmitted “in poorly ventilated and/or crowded indoor settings,” saying this is because “aerosols remain suspended in the air or travel farther than 1 meter.”