Mumbai. Jakarta. Manila.

All of these mega-metropolises share a global risk, scientists say: they could be breeding grounds for the next pandemic.

A group of international researchers, led by scientists at the University of Sydney, have mapped out the cities that offer the ripest conditions for the incubation of viruses that could jump from animals to humans—and then spread around the world.

The researchers tracked the risks based on three factors: the first was identifying cities where intense urban life collides with both domesticated animals and rich biodiversity, raising interactions between humans, farmed animals, and wildlife, often due to disruption like deforestation.

This increases the risks of zoonotic disease outbreaks in which, for example, a virus makes the jump from a mammal or bird species to humans. Well before the arrival of COVID-19, virologists and other scientists have been sounding the alarm that our modern world creates ideal conditions for just such an outbreak, as Fortune wrote in April 2020.

But to create a global pandemic, the Sydney researchers researched an additional two factors.

The second is tracking which of those super cities are globally connected—not just through international airports and aviation routes—allowing the virus to be spread quickly. The third factor is an insufficient public healthcare infrastructure (measured, in this case, by rates of infant mortality), which increased the risk that a virus in circulation won't be identified and reported straight away.

Combining those factors, the researchers found that 20% of the world's most-connected cities are at risk of such disease spreads. Another 14-20% of those cities have poor health infrastructure, with the majority in South and Southeast Asia, and Sub-Saharan Africa. The cities with the highest risks including Mumbai, Delhi, Jakarta, Singapore, and Kunming, in China.

The researchers broke the risks down into three levels: a yellow, orange and red rating system with red the highest risk, yellow the lowest.

Low- and -middle income countries are highly represented in "red" due to the healthcare risks. But some high-income cities—the researchers cite San Francisco, and Adelaide, Australia as two—can be found in the yellow tier, said Michael Walsh, an infectious disease epidemiologist at the University of Sydney and one of the authors of the study, in a release. That's because of the extreme impact more affluent cities can have on local wildlife, Walsh said.

Addressing these risks would require a holistic approach, he said, one that looks at wildlife conservation and animal husbandry alongside human healthcare systems and monitoring and surveillance at airports—a huge cost, but a necessary one.

"Given the overwhelming risk absorbed by so many of the world's communities and the concurrent high-risk exposure of so many of our most connected cities, this is something that requires our collective prompt attention."