Viruses vary in how they infect. The new coronavirus -- unlike its cousins Sars, or severe acute respiratory syndrome, and Mers, or Middle East respiratory syndrome -- spreads as easily as a common cold.
And it's almost certainly being spread by people who show such mild symptoms that no one can tell, said Dr Amesh Adalja of the Johns Hopkins Center for Health Security.
"If that's the case, all of these containment methods are not going to work," Adalja said. "It's likely mixed in the cold and flu season all over the place, in multiple countries" and gone unnoticed until someone gets severely ill.
These milder symptoms are good news "in terms of not as many people dying," said Mackay, of Australia. "But it's really bad news if you are trying to stop a pandemic," he added. (AP)