Ma Xiaowei, the director of China's National Health Commission, speaks at a press conference about a virus outbreak at the State Council Information Office in Beijing, Sunday. AP Photo/Mark Schiefelbein

China’s health minister Ma Xiaowei had some very bad news Sunday about the Wuhan coronavirus: He said people can spread it before they have symptoms.

A veteran adviser for a US health agency called the news a "game changer."

“When I heard this, I thought, ‘oh dear, this is worse than we anticipated.’ It means the infection is much more contagious than we originally thought,” said Dr. William Schaffner, a longtime adviser to the Centers for Disease Control and Prevention.

Schaffner added that this new piece of information called into question the current US practices for containing the virus so it doesn’t spread beyond the three cases that have already appeared in Washington state, California, and Illinois.

Schaffner, an infectious disease expert at Vanderbilt University Medical Center said that if Ma is correct, “we’re going to have to re-evaluate our strategy, that’s for sure."

And if so – and information about this virus is constantly evolving -- for about two weeks, people who don’t even know they’re sick can spread the virus, which has killed more than 50 people in China and infected thousands.

The disease has spread as far as France, Canada, and the US, which now has three cases in California, Washington state, and Illinois.

In a press briefing Friday, Dr. Jennifer Layden, an epidemiologist with the Illinois Department of Health, said the woman with the Wuhan coronavirus in her state had not been sick while traveling from Wuhan to the US on January 13, and “based on what we know now about this virus our concern for transmission before symptoms develop is low so that is reassuring.”

She explained that officials were following “close contacts” of the patient. She did not mention following other kinds of contacts, such as the other passengers on the woman’s January 13 flight.

This post has been updated to clarify Ma Xiaowei's honorific.