SEOUL - The number of South Koreans infected with the Middle East Respiratory Syndrome (MERS) rose to 18 Monday as three more cases were tested positive, the health ministry said.

People who came into close contact with the South Korean MERS patient arrive at Lady MacLehose Holiday Village in Sai Kung, where they will spend two weeks in quarantine, in Hong Kong, China May 30, 2015.?[Photo/Agencies]

Three persons who had close contact with the patient zero were confirmed positive for the deadly viral disease, according to the Ministry of Health and Welfare. No tertiary infection case has been reported yet.

The 16th and 18th infectees are a 40-year-old man and a 77-year- old woman who were hospitalized at the same ward with the first patient.

The 17th case is a 45-year-old man, son of a patient who shared the same hospital ward with the patient zero.

All the new cases were infected directly from the first patient for three days from May 15 at the hospital, which the patient zero visited.

The three infectees were at first excluded from the ministry's list of people required to be isolated and observed, but they were tested positive after the renewed epidemiological survey for people staying with the first patient at the second hospital.

Among the combined 18 infectees, 15 cases were infected from the first patient at the second hospital, and two cases were medical staff members, including a doctor and a nurse who treated the patient zero at another hospital.

The 15 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 MERS is a respiratory illness caused by a new type of corona-virus that is similar to the virus causing Severe Acute Respiratory Syndrome (SARS). There is no vaccine or treatment for the disease, with its fatality rate reaching 40.7 percent.

The first case was spotted in Saudi Arabia in 2012. The World Health Organization has reported more than 1,000 cases of MERS globally and more than 400 deaths.