Officials in South Korea report that five people have now succumbed to Middle East Respiratory Syndrome (MERS).
The latest fatality was a 75-year-old man who had been in the same Seoul hospital emergency room where a total of 17 people, including two medical staff, are believed to have been infected with MERS, South Korea's health ministry said.
Hundreds of schools have closed and nearly 2,000 people are under quarantine.
South Korean authorities will track the mobile phones of hundreds of people under quarantine to prevent the spread of the virus.
"Please understand this is an unavoidable measure for the sake of our neighbours and families," Deputy Prime Minister Choi Kyung Hwan told a news briefing, as he announced the plan to track the mobile phones of people under quarantine to make sure they stay at home.
It's believed that 64 people have now been infected by the virus. Kyung Hwan denied that MERS will spread further throughout the country.
"So far, all the MERS cases have been hospital-associated, and there has been no case of an infection in other social settings. We think we have a chance at putting the outbreak under total control," he said.
The UN health agency says there's no evidence yet in South Korea of "sustained transmission in the community".
MERS was first identified in humans in 2012. It is caused by a coronavirus from the same family as Severe Acute Respiratory Syndrome (SARS). But MERS has a much higher death rate at 38%, according to World Health Organisation figures.
While the virus has no vaccine, health experts believe it spreads through close contact with infected people and not through the air.
Scientists say the worst-case scenario is if the virus changes and spreads rapidly, as Sars did in 2002-2003 when it killed about 800 people around the world.
South Korea's new cases bring the total number of MERS cases globally to about 1,208, based on WHO data, with at least 444 related deaths.