4 surprising reasons scientists think asymptomatic coronavirus cases are so common - Yahoo News

The coronavirus is a serious, often-deadly pathogen, yet the Centers for Disease Control and Prevention estimates 40 percent of all cases are asymptomatic.

One research group found that, among uninfected blood samples donated to a blood bank between 2015 and 2018, a "remarkable" 40 to 60 percent recognized the coronavirus, suggesting some people may have an immune response based on memory of other, less potent coronaviruses.

On the Diamond Princess, where masks weren't used, 47 percent of the positive cases were asymptomatic, whereas an Antarctic-bound Argentine cruise ship that had a similar outbreak, but provided masks to all passengers and crew, saw an 81 percent asymptomatic rate.