What happened after the outbreak went global: The real contagious threat was ignored.On the precipice of a pandemic, too many important officials failed to understand how the virus was spreading, despite emerging evidence, keeping them from effectively limiting its spread and costing thousands of lives.On Feb. 3, 2020, the cruise ship Diamond Princess was ordered to stay in Yokohama harbor, in Japan, two days after a passenger who had disembarked in Hong Kong tested positive for Covid. After 10 other people on the ship were found to be infected, the ship was quarantined. Eventually there would be 712 cases, about 19 percent of those on board, with 14 deaths.Nine public health workers attending to the ship were infected. It seemed quite unlikely, the Japanese virology professor Hitoshi Oshitani noted, that all these professionals with expertise in infection control had failed to take the recommended precautions.At that point the guidelines from the W.H.O. and the Centers for Disease Control and Prevention were based on the assumption that this virus was spread by large droplets from the nose and mouth that quickly fell to the ground or to surfaces, because of their size. People were advised to keep enough distance from others to stay out of the range of these droplets, and to wash their hands in case they picked them up from surfaces.If the workers became infected despite those precautions, and if passengers were infected even when they were quarantined, Oshitani suspected that the virus was probably spread by airborne transmission of tiny particles — aerosols — that could spread more widely, float around and concentrate, especially indoors.This case for aerosol spread strengthened after 61 people attended a choir practice in Skagit, Wash., on March 10, 2020. The church followed droplet-based guidance by propping the door open so nobody would touch the door knob and avoiding handshakes or hugs. No one was six feet in front of the person suspected to have been the single initial source. Nevertheless, 52 people — 85 percent of those present — became infected.Many Western experts, including in the United States and Europe and at the W.H.O., discounted these and other evidence of airborne transmission. Countries like the United States did not require masks to limit airborne spread but worried instead about germs spreading on people’s mail and groceries.