Hong Kong (CNN) The World Health Organization warned that the MERS outbreak in South Korea is likely to grow, as the number of people under quarantine crept up to 1,369 on Wednesday.

The Korean Centers for Disease Control and Prevention confirmed five new cases -- increasing the number of people with the disease to 35. These new cases were contracted within hospitals.

So far, three people have died after contracting the respiratory virus in South Korea, the country's Health Ministry said Thursday, in the largest MERS outbreak outside Saudi Arabia.

The first case, concerning a man who returned to South Korea after traveling to Saudi Arabia, Qatar, the UAE and Bahrain, was reported on May 20. The person had not been ill during his travels, according to the World Health Organization.

More than 900 schools have shut to prevent the spread of the virus, according to South Korea's education ministry.

The extent of the outbreak in South Korea has taken many by surprise -- mainly because the virus has not been shown to spread easily between humans and the health care system in the country is considered to be sophisticated and modern.

South Korean President Park Geun-hye acknowledged problems in the country's early response earlier this week.

"Initial reaction for new infectious diseases like MERS is very important, but there were some insufficiency in the initial response, including the judgment on its contagiousness," she said.

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She convened an emergency MERS meeting on Wednesday.

MERS is in the same family of viruses as SARS (severe acute respiratory syndrome) as well as the common cold. However, MERS does not spread easily between humans -- as far as scientists know at this point.

"So far, the virus has been circulating in humans for three years," said Dr. Leo Poon, a virology expert at the School of Public Health at the University of Hong Kong, who worked on the SARS outbreak more than a decade ago. "We found little transmission in human. We know there is human-to-human transmission, but it's not sustainable."

Then why is it spreading in South Korea?

Since MERS, short for Middle East respiratory syndrome, was first reported in Saudi Arabia in 2012, international cases have largely been confined to travelers bringing the virus back to other countries and infecting one or two others. There have been deaths in countries like Oman, Algeria, and Malaysia -- but none of them had additional infections to the extent of South Korea.

"This is quite unusual. I think this is the only country, apart from those in the Middle East, that has such a number of cases," said Poon. "It's not entirely surprising. In the Middle East, people in Saudi Arabia had hospital outbreaks where a few people got infected. It's a similar situation at the moment."

In early 2013, 23 MERS cases in eastern Saudi Arabia were linked to a single outbreak extending through four health care facilities.

Similarly, the vast majority of the South Korean cases have been linked to infections from hospitals.

Another factor for the spread in South Korea could be the fact that family members often stay with patients in their hospital rooms to watch and care for their loved ones.

"With the hospital culture here, the family does a lot of the nursing. For general patients on the ward there are fewer nurses than we are accustomed to in the West," said Dr. John Linton of Yonsei University's Severance Hospital in Seoul. "They would have been in close proximity to other patients."

How did it start in South Korea?

The first patient, a 68-year-old man, had traveled to four Middle Eastern countries before returning to South Korea on May 4. During his flight, he did not have any symptoms.

As he started getting sick a week later, the Korean sought treatment at two clinics and two hospitals -- "creating multiple opportunities for exposure among health care workers and other patients," according to the WHO. MERS was not suspected and health care workers did not treat the first patient in isolation.

As a result, the MERS cases in South Korea span patients from several health care facilities. Health officials have not identified the hospitals, but 22 of the current cases are related to those who were at what's being called "Facility B." That hospital has closed voluntarily.

"Given the number of clinics and hospitals that cared for the index case, further cases can be expected," the WHO stated in a situation report on Wednesday.

Some of the infected people occupied the same room as the first patient and others had been in the same ward for times ranging from five minutes to several hours, according to the WHO.

How does MERS spread?

Concern about the virus is gripping many in South Korea, with schools shutting and the increased use of face masks and hand sanitizers.

The virus acts like a cold and attacks the respiratory system, the Centers for Disease Control and Prevention has said. But symptoms, which include fever and a cough, are severe and can lead to pneumonia and kidney failure.

MERS spreads from close contact with an ill person, such as living or caring for them.

About three to four out of every 10 people reported with MERS have died. But the people who died often had underlying medical conditions that made them more vulnerable.

The two patients who died in South Korea had chronic obstructive pulmonary disease and heavy asthma.

As of Wednesday, there have been 1,179 confirmed cases of MERS reported to WHO since 2012, and at least 442 cases were fatal. Cases have been reported in 25 countries , with China and South Korea joining the ranks only last month, WHO said.

MERS has been linked to camels and it's possible that some people became infected after coming into contact with camels, but it's not completely clear.

There are no vaccines and no cures.

To prevent MERS, the CDC recommends everyday hygiene practices like hand-washing, covering coughs and sneezes, and avoiding personal contact with sick people.