REPUBLIKA.CO.ID, CHICAGO - Scientists leading the fight against Middle East Respiratory Syndrome said the next critical front would be understanding how the virus behaved in people with milder infections, who might be spreading the illness without being aware they have it.

Establishing that may be critical to stopping the spread of MERS, which emerged in the Middle East in 2012 and has so far infected more than 500 patients in Saudi Arabia alone. It kills about 30 percent of those who are infected.

It was becoming increasingly clear that people could be infected with MERS without developing severe respiratory disease, said Dr David Swerdlow, who heads the MERS response team at the US Centers for Disease Control and Prevention.

"You don't have to be in the intensive care unit with pneumonia to have a case of MERS," Swerdlow told Reuters. "We assume they are less infectious (to others), but we don't know."

The CDC has a team in Saudi Arabia studying whether such mild cases are still capable of spreading the virus. Swerdlow is overseeing their work from Atlanta. They plan to test the family members of people with mild MERS, even if these relatives don't have any symptoms, to help determine whether the virus can spread within a household.

Milder symptoms played a role in the second US case of MERS, a man who started having body aches on a journey from Jeddah on Saudi Arabia's Red Sea coast to the United States.

It took the patient more than a week before he sought help in an emergency department in Orlando, Florida. Once he arrived, he waited nearly 12 hours in the ER before staff recognized a MERS link and placed him in an isolation room. The patient did not have signs of a respiratory infection, not even a cough.

Cases of the disease, which causes coughing, fever and sometimes fatal pneumonia, have nearly tripled in the past month and a half, and the virus is moving out of the Arabian peninsula as infected individuals travel from the region. Since late April, the first two cases of MERS have been reported on US soil. Dutch officials reported their first two cases this week. Infections have also turned up in Britain, Greece, France, Italy, Malaysia and elsewhere.

Since MERS is an entirely new virus, there are no drugs to treat it and no vaccines capable of preventing its spread. It is a close cousin of the virus that caused Severe Acute Respiratory Syndrome or SARS, which killed around 800 people worldwide after it first appeared in China in 2002.

Because MERS patients can have "mild and unusual symptoms," the World Health Organization advises healthcare workers to apply standard infection control precautions for all patients, regardless of their diagnosis, at all times.

"Asymptomatic carriers of diseases can represent a major route for a pathogen to spread," said Dr Amesh Adalja of the University of Pittsburgh Medical Center. "Just think of Typhoid Mary," he said, referring to the asymptomatic cook who spread typhoid fever to dozens of people in the early 20th century.

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