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Mr Cao and his wife live in the north-eastern province of Liaoning, China's former industrial heartland.

This is a different world from Liaoning - the shiny new buildings that are rising up host tech companies such as Alibaba and Ant, a magnet for ambitious, young entrepreneurs.

Back in Liaoning, the wood smoke rises from chimneys, signalling lunchtime.

In October, researchers spotted Chinese-operated buses and vans transporting deportees from detention centres in China's Jilin and Liaoning provinces to five crossing points along the border with North Korea.

“Chinese authorities advised that there has been no detection of any unusual or novel pathogens or unusual clinical presentations, including in Beijing and Liaoning, but only the aforementioned general increase in respiratory illnesses due to multiple known pathogens,” the WHO said in a statement last month.