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By Julie Steenhuysen CHICAGO (Reuters) - Scientists leading the fight against Middle East Respiratory Syndrome say the next critical front will be understanding how the virus behaves in people with milder infections, who may 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 is becoming increasingly clear that people can be infected with MERS without developing severe respiratory disease, said Dr David Swerdlow, who heads the MERS response team at the U.S. 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. 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 U.S. 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. NOT EVEN A COUGH Milder symptoms played a role in the second U.S. 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.[ID:nL1N0O002W] Dr Kevin Sherin, director of the Florida Department of Health for Orange County, believes that made it less likely that he could spread the infection. Hospital workers have tested negative, but the health department and the CDC are still checking on hundreds of people who might have been in contact with the patient. A CDC study published earlier this week looked at some of the first cases of MERS that occurred in Jordan in 2012. Initially, only two people in that outbreak were thought to have MERS. When CDC disease detectives used more sensitive tests that looked for MERS antibodies among hospital workers, they found another seven people had contracted MERS and survived it. That suggests there may be people with mild cases "that can serve as a way for the virus to spread to other individuals, which makes it a lot harder to control," Adalja said. Scientists are especially concerned because a lot of recent cases of MERS are among people who did not have contact with animals such as camels or bats that are believed to be reservoirs for the virus. "If they don't have animal contact, where do they pick it up? Potentially, asymptomatic cases," said Dr Michael Osterholm, an infectious disease expert from the University of Minnesota. (Additional reporting by Sharon Begley in New York; Editing by Michele Gershberg)