With billions of people moving around the planet every year, the U.N. agency said in its annual World Health Report: "An outbreak or epidemic in one part of the world is only a few hours away from becoming an imminent threat somewhere else."

WHO director-general Margaret Chan said mass travel could facilitate the rapid spread of infectious diseases.

"No country can shield itself from invasion by a pathogen incubating in an airline passenger or an insect hiding in a cargo hold," Chan told reporters.

 Source : Reuters