Iranian officials trace the origins of the country’s coronavirus epidemic to the holy city of Qom, home to dozens of seminaries and religious shrines—but also a number of Chinese-backed infrastructure projects built by scores of workers and technicians from China.

This critical link to China, centered in Qom, has helped keep Iran’s economy alive in the face of American sanctions. And it is now being stress-tested by the coronavirus. The exact route of the virus is unclear. But Iran’s strategic partnership with Beijing has created a constellation of potential contacts that helped unleash the illness, called Covid-19.

“China has been the trading partner of last resort but, in this case, it has turned into a very toxic bomb,” said Sanam Vakil, deputy Middle East director at Chatham House, a think tank in London.

China Railway Engineering Corp. is building a $2.7 billion high-speed rail line through Qom. Chinese technicians have been helping refurbish a nuclear-power plant nearby. There are also Chinese religious students studying at Qom’s seminaries.

Iranian health officials have said the source of the outbreak is likely either Chinese workers in Qom or an Iranian businessman from Qom who travelled to China. Iranian officials haven’t identified the businessman by name but say he travelled from China to Qom through an indirect flight.