A single patient infected 82 people with the deadly Mers virus in an overcrowded South Korean emergency room in 2015, according to a scientific investigation released on Saturday.
The study, published in the medical journal The Lancet, maps a lethal outbreak of Middle Eastern Respiratory Syndrome (Mers) in South Korea that caused 186 known infections in all, including nearly 40 fatalities.
The case is the most prolific transmission of Mers virus from one patient outside the Middle East.
As long as the highly-contagious respiratory disease -- with a 30-40% mortality rate -- continues to circulate in the Middle East, governments and health care providers should be prepared for sudden outbreaks elsewhere, the researchers warn.
"This study is the first to document the spread of Mers virus through a hospital," Doo Ryeon Chung and Yae-Jean Kim of Samsung Medical Centre -- where the outbreak occurred -- said in a statement.
"Our results show the increased potential of Mers virus infection from a single patient in an overcrowded emergency room."
The outbreak in South Korea began with a 68-year-old man -- known as "patient 1" -- who had travelled to Bahrain, the United Arab Emirates, Saudi Arabia and Qatar in the spring of 2015.
Correctly diagnosed, he was admitted to Samsung and isolated from other patients.
But unknown to doctors and health officials, he had already infected several other people, including a 35-year-old man with whom he shared a ward in another health facility.
It was this younger man, "patient 14," who later spread the virus through Samsung's emergency ward.
The researchers estimate that nearly 1,600 people were exposed to patient 14 in the emergency room.
Of the 82 who were infected, 33 were patients, eight were health care workers, and 41 were visitors.
Patients staying in the same zone of the emergency room had a 20% chance of contracting Mers, the study found.