New coronavirus has now reached more than 20 countries from China, where the epidemic or disease outbreak began over a month ago.
Experts are worried about how much further it could spread and how many people will get sick.
A truly global outbreak or pandemic has not been declared yet.
But officials are preparing for the possibility that this could be the next pandemic that the world will have to face.
There have been more than 17,000 confirmed coronavirus cases and around 360 deaths, mostly in China.
Outside China, there are more than 150 confirmed cases of the virus - and one death, in the Philippines.
"If we invest in fighting at the epicentre, at the source, then the spread to other countries is minimal and also slow," Dr Ghebreyesus told a WHO executive board meeting on Monday.
Each pandemic is different and, until a virus starts circulating, it is impossible to predict its full effects.
Experts suspect that coronavirus may be less deadly than some other recent disease outbreaks, such as Sars.
The decision by the World Health Organization to declare the situation a Public Health Emergency of International Concern (PHEIC) was taken primarily because of the signs of human-to-human transmission outside China, and what might happen if the virus were to spread in a country with a weaker health system.
WHO says although countries should take steps to prevent and limit further spread of the virus, there is no reason yet for measures which unnecessarily interfere with international travel and trade.