It began in 2002 with SARS, a rare coronavirus that first broke out in Asia. By early 2003, the virus had spread to at least two continents, mostly hitting Western Asia, but then faded away into obscurity.

Then in May 2016, a strain of the virus — with a deadlier virus but less severe symptoms — was discovered in Saudi Arabia.

In May 2017, a tiny virus surfaced in Sweden, where a traveler infected two people who later died. But less than three months later, scientists in the U.S. were able to successfully eradicate this new coronavirus, also known as MERS-CoV.

After the relatively small outbreak, the biggest worry was that the virus would reappear in humans. Some researchers continued to worry that it had the potential to cause a pandemic, or an airborne infection that jumped from animals to humans.

The increased efforts at ferret testing after MERS-CoV emerged could help minimize the risk, said Dr. Jonathan Ball, an infectious disease specialist at the University of Edinburgh. It also shows the possibility that current biosecurity measures could protect society from biological threats, he said.

“These investigations are incredibly important,” he said. “We know that public health intervention in the form of control measures has been found to reduce the number of human cases in Australia and now in Sweden, which gives us an important reference point.”

During 2016, 829 new cases of MERS-CoV, including two in the U.S., were reported to the World Health Organization.

The Dutch health ministry recently said the virus was found in surface tap water, toothpaste and soil, raising doubts about its ultimate source. But it was difficult to trace, Ball said.

This story was reported by The Times staff.