A deadly new virus that has killed more than half the people who catch it is now a global threat, the head of the World Health Organization warned.

Medical experts are stumped by the quick-spreading disease that attacks the respiratory system and has spread from the Mideast to Britain, Germany, France and Pakistan.

“We do not know where the virus hides in nature. We do not know how people are getting infected,” WHO general director Margaret Chan told a meeting in Geneva on Monday.

“Until we answer these questions, we are empty-handed when it comes to prevention,” she added.

Authorities said the virus is similar to SARS (severe acute respiratory syndrome), which triggered an epidemic that killed more than 800 people in 2003.

The new disease, called MERS-CoV, for Middle East respiratory syndrome coronavirus, has killed 27 out of 49 people infected.