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-traveling 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, named A World At Risk, said current efforts to prepare for outbreaks in the wake of crises such as Ebola 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 help the disease spread faster.
With vast numbers of people crossing the world on planes every day, an equivalent air-borne outbreak now could spread globally in less than 36 hours and kill an estimated 50million to 80million people, they said.
The GPMB report warned: 'There is a very real threat of a rapidly moving, highly lethal pandemic of a respiratory pathogen killing 50 to 80million people and wiping out nearly five percent of the world's economy.
'A global pandemic on that scale would be catastrophic, creating widespread havoc, instability and insecurity. The world is not prepared.'
In the case of a pandemic, many national health systems - particularly in poor countries - would collapse, they said.
'Poverty and fragility exacerbate outbreaks of infectious disease and help create the conditions for pandemics to take hold,' said Axel van Trotsenburg, acting chief executive of the World Bank and a member of the panel.
The report set out a series of actions the international community must take to protect people around the world in the event of an illness spreading out of control.
Tedros Adhanom Ghebreyesus, director-general of the WHO, called on governments to 'heed the lessons these outbreaks are teaching us' and to 'fix the roof before the rain comes'.
In their recommendations, the team said governments must ringfence money for putting preparations in place and do routine simulation exercises.
The G7, G20 and G77 states should set an example for the rest of the world to follow, they added, and all parties must 'prepare for the worst'.
They also called for more private investment into countries' pandemic preparations and said the UN must do more to coordinate responses across international borders.
The WHO also warned earlier this year that another pandemic of flu - which is caused by airborne viruses - is inevitable, and said the world should prepare for it.