A higher dose is clearly worse, though, and that may explain why some young health care workers have fallen victim even though the virus usually targets older people.

The crucial dose may also vary depending on whether it’s ingested or inhaled.

People may take in virus by touching a contaminated surface and then putting their hands on their nose or mouth. But “this isn’t thought to be the main way the virus spreads,” according to the Centers for Disease Control and Prevention.

That form of transmission may require millions more copies of the virus to cause an infection, compared to inhalation.

Coughing, sneezing, singing, talking and even heavy breathing can result in the expulsion of thousands of large and small respiratory droplets carrying the virus.