January 24, 2021 A year after the emergence of the disease that has killed 2 million people, humans keep underestimating the SARS-CoV-2 virus. This winter, Covid-19 is turning out to be even more dangerous than epidemiologists and public health officials had feared—and not just because of the more contagious variants now making their way around the globe. As recently as October, Nature reported it was “too soon to say whether Covid is seasonal like the flu.” Evidence hinted that winter weather could increase transmission of the virus: In the lab, the virus persisted under cold, dry conditions and was inactivated by the ultraviolet rays in sunlight.