If a disease like the Spanish flu emerged today it could kill 80 million people, experts say.
A report from the World Health Organization's (WHO) new Global Preparedness Monitoring Board (GPMB) puts the blame on governments that "quickly forget", climate change and "misinformation" spread by social media - such as hysteria over vaccines - eroding trust in medical professionals and scientists.
"The world is not prepared for a fast-moving, virulent respiratory pathogen pandemic," the report, published this week, reads.
In 1918, Spanish flu infected a third of the world's population and killed about 50 million people - and that was before people were able to fly anywhere on the planet in under 36 hours.
The report's authors looked at how authorities responded to the 2009 H1N1 influenza pandemic and the 2014-2016 Ebola outbreak, and concluded that despite improvements in treatment since 1918, a similar outbreak now could kill even more, the report says.
"While disease has always been part of the human experience, a combination of global trends, including insecurity and extreme weather, has heightened the risk," said GPMB co-chairs Gro Harlem Brundtland (former WHO director-general) and Elhadj As Sy, the secretary-general of the International Federation of Red Cross and Red Crescent Societies.
"Disease thrives in disorder and has taken advantage - outbreaks have been on the rise for the past several decades and the spectre of a global health emergency looms large."
A Spanish flu-style outbreak would also have the potential to knock 5 percent off the global economy, but the damage won't be spread out - instead concentrated on poorer countries.
"Outbreaks hit lower-resourced communities much harder given their lack of access to basic health services, clean water and sanitation; this will aggravate the spread of any infectious pathogen," said Dr Brundtland and Sy.
"Disease amplifiers, including population growth and resulting strains on the environment, climate change, dense urbanisation, exponential increases in international travel and migration, both forced and voluntary, increase the risk for everyone, everywhere."
A virus would be even deadlier if it was deliberately released, the report says. Anthrax bioterrorism is noted as a "deliberately emerging" risk in the US.
"In addition to the need to decide how to counter the pathogen, security measures would come into play limiting information-sharing and fomenting social divisions."
Measles outbreaks have increased in recent years, and the WHO recently declared anti-vaxxers - people opposed to vaccines for unscientific or fraudulent reasons - as one of the top 10 threats to global health.
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"Trust in institutions is eroding," the report says. "Governments, scientists, the media, public health, health systems and health workers in many countries are facing a breakdown in public trust that is threatening their ability to function effectively. The situation is exacerbated by misinformation that can hinder disease control communicated quickly and widely via social media."
New Zealand is noted in the report as being unlikely to suffer too much economically in the event of an outbreak, along with the US, Canada, Europe, Saudi Arabia, Oman, Japan, Chile and Uruguay.
The Spanish flu was so deadly because it turned the body's immune system against its host. This resulted in massive death tolls among young adults - those with the strongest immune systems - unlike most diseases, which typically hit children and the elderly hardest.
No one knows where it originated - it got the name 'Spanish flu' because reports of its spread across most of Europe and the US were censored to keep wartime morale up. Papers were free to report on its spread in neutral Spain, however.
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The closest example given in the new report of a newly emerging disease in our part of the world is Australia's Hendra virus, which mainly infects fruit bats and horses, but can make the leap to humans.
"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," said Dr Brundtland and Sy.
"It is well past time to act."