London: The World Health Organisation (WHO) has put scientists and health workers around the globe on alert for a new and potentially deadly pathogen - Disease X.

Each year the Geneva-based organisation, which is charged with monitoring and safeguarding world health, convenes a high-level meeting of senior scientists to list diseases that pose a serious risk of prompting a major international public health emergency.

In previous years the list has been confined to known killers such as Zika, Lassa fever, which is sweeping Nigeria, and Ebola, and which killed more than 11,000 people in an epidemic in west Africa between 2013 and 2016.

However, this year a mystery pathogen has been added as the ninth item on the watch list for the first time. "Disease X represents the knowledge that a serious international epidemic could be caused by a pathogen currently unknown to cause human disease," said the WHO in a statement.

"History tells us that it is likely the next big outbreak will be something we have not seen before," said John-Arne Rottingen, chief executive of the Research Council of Norway and a scientific adviser to the WHO committee.