Details that have emerged about the new virus from China reveal how challenging it may be to control the outbreak.
The virus has infected more than 17,000 people and spread to two dozen countries. China says it's now caused 362 deaths.
Scientists have found that the virus can spread person to person, even if someone is showing no symptoms.
The next in line can continue to pass it on.
The incubation period is up to two weeks, so people may not know where or when they picked it up.
The risk to the public outside of China, where most cases are, is still considered low.