Seeking to explain findings from multiple countries earlier this year that as much as half the population may have some built-in immunity to COVID-19, researchers studied blood samples taken from healthy subjects collected before the pandemic. They found that many patients had T cells that recognized and help fight COVID-19 without having ever been exposed to it. They believe that the similarities between the new coronavirus and the cold viruses cause some people to develop these T cells protective against COVID-19. It may also explain why some people get the virus and have few or no symptoms, or don’t get the virus at all.