China's new deadly virus is more difficult to contain than other pandemics because those infected may not get a fever, a symptom governments around the world are using to screen for the pathogen.

China's National Health Commission told local governments and hospitals on Jan. 18 that some of those with the virus may not have a fever and the pathogen's incubation period could be up to two weeks, according to people familiar with the matter, who asked not to be named discussing the situation.

A spokesman for the NHC said that the understanding of the new virus and its symptoms changes over time and that doctors have observed some patients with low or even no fever as the number of infections has increased.