A flu-like illness could travel around the world in 36 hours and kill 80million people, experts have warned.
A century ago the Spanish flu pandemic infected a third of the world's population and killed 50million people.
If a similar outbreak were to happen with today's constantly-travelling population, the effects could be even worse, a report has suggested.
The Global Preparedness Monitoring Board (GPMB), a team of health experts led by a former chief of the World Health Organization, has produced the report to try and spur world leaders into action.
'The threat of a pandemic spreading around the globe is a real one,' the group said in a report released today.
'A quick-moving pathogen has the potential to kill tens of millions of people, disrupt economies and destabilise national security.'
The report A World At Risk listed dozens of illnesses which the experts suggested had the potential to trigger an outbreak which could spiral out of control, among them plague, Ebola, Zika virus and Dengue
The report, named A World At Risk, said current efforts to prepare for outbreaks are 'grossly insufficient'.
The GPMB is headed by Dr Gro Harlem Brundtland, the former Norwegian prime minister and director-general of the WHO, and Mr Alhadj As Sy, the secretary-general of the International Federation of Red Cross and Red Crescent Societies.
It said in its report that recommendations it made in an earlier report have been largely ignored by world leaders.
'Many of the recommendations reviewed were poorly implemented, or not implemented at all, and serious gaps persist,' the GPMB wrote.
'For too long, we have allowed a cycle of panic and neglect when it comes to pandemics: we ramp up efforts when there is a serious threat, then quickly forget about them when the threat subsides. It is well past time to act.'
The report contained a map of the world with a list of possible infections which could trigger the hypothetical outbreak.
These were split into 'newly emerging' and 're-emerging/resurging'.
Among the former were the Ebola, Zika and Nipah viruses, and five types of flu.
And the latter included West Nile virus, antibiotic resistance, measles, acute flaccid myelitis, Yellow fever, Dengue, plague and human monkeypox.
The report referenced the damage done by the 1918 Spanish flu pandemic and said modern advances in international travel would