The South Korean government says a doctor infected with Middle East Respiratory Syndrome (MERS) has come into contact with more than 1,000 people.
The doctor is said to have attended large-scale meetings while infected, Al Jazeera's Harry Fawcett reported from Seoul, the capital, on Thursday.
On Friday, South Korea reported its fourth death from an outbreak of the virus that has infected dozens of people, seen hundreds of schools closed and caused thousands to cancel travel plans.
The most recent patient to die was a 76-year-old man who had been in the same ward as other MERS patients and had been suffering from various ailments including cancer, the health ministry said.
An 82-year-old man was diagnosed with MERS in a posthumous test after he died in hospital on Wednesday night.
He was originally being treated for asthma and pneumonia but was placed under quarantine after other patients in his ward tested positive for MERS.
The latest case brought the total number of people diagnosed with the potentially deadly virus in South Korea to 41, in the largest outbreak outside Saudi Arabia.
More than 900 schools, from kindergartens to colleges, have now shut their gates and the government's MERS hotline took more than 3,000 calls on Wednesday as public fears grew.
Before Thursday, a 58-year-old woman and a 71-year-old man had died in South Korea from the disease, which has no cure or vaccine.
The first case, reported on May 20, was of a 68-year-old man diagnosed after a trip to Saudi Arabia.
Since then, more than 1,660 people who may have been exposed to the virus have been placed under varying levels of quarantine.
While around 160 were isolated at state-designated facilities, most were told to stay home and strictly limit their interactions with other people.
In Seoul, growing public concern has been reflected in the daily increase in the number of commuters wearing face masks on buses and subways.
The anxiety has been exported, with the Korea Tourism Organisation (KTO) reporting on Thursday that around 7,000 tourists - mostly from China and Taiwan - had cancelled planned group trips to South Korea.
"A mass cancellation of this scale is very unusual ... and many travellers cited the MERS outbreak as the main reason," a KTO spokesperson told the AFP news agency.
The military has also been affected with more than 20 symptomatic soldiers quarantined, including six who had contact with a South Korean air force officer recently diagnosed with MERS.
Source: Al Jazeera and agencies