SEOUL, May 31 -- The number of South Koreans infected with Middle East Respiratory Syndrome (MERS) has risen to 15 after two new cases were detected, the health ministry said Sunday.

Two men in their 30s, who had close contact with the patient zero, were confirmed positive for the deadly viral disease, according to the Ministry of Health and Welfare.

The 14th case is a 35-year-old patient who was hospitalized at the same ward with the patient zero, and the 15th case is a 35- year-old man who visited her mother being accepted to the same ward with the first patient.

The 13th case was tested positive on Saturday as the 49-year- old man nursed his wife, the 12th infectee who had been hospitalized at the same ward with the patient zero.

No tertiary infection case has been reported yet as 14 others were all infected directly from the 68-year-old first patient.

Among them were 12 people who had contacts with the first patient at the same hospital ward and two medical staffs, including a doctor and a nurse who treated the patient zero.

Health Minister Moon Hyung-pyo said at a meeting with representatives of doctors, nurses and hospitals Sunday that the country would make all-out efforts to prevent the tertiary infection, apologizing for the spread of the MERS.

All the 14 infectees had close contact with the patient zero for three days from May 15. Given the latent period of two weeks, the number of infectees is not likely to rise further from June 3 unless any tertiary infection happens.

The first patient has been isolated since May 20 when he was tested positive for the MERS after returning home on May 4 from his Middle East travel.

The patient zero's wife became the second patient who contracted the deadly virus.

The third infectee was a 76-year-old man who shared a hospital room with the first patient, and his daughter who attended to the third patient became the fourth.

A South Korean man, who had close contacts with the third and fourth patients, went to China for a business trip on Tuesday. The 44-year-old man was tested positive for the MERS, according to the Chinese health authorities.

He flew from South Korea to Hong Kong via the flight of the Asiana Airlines on Tuesday and entered Huizhou city through Shenzhen. He had close contact with his father and sister, the third and fourth patients infected with the MERS corona virus.

The MERS is a respiratory illness caused by a new type of corona-virus. The first case was spotted in Saudi Arabia in 2012. There is no vaccine or treatment for the disease, with its fatality rate reaching 40.7 percent.