(CNN) -- The spread of the potentially fatal Middle East respiratory syndrome has become more serious and urgent, the World Health Organization said Wednesday.
But it does not constitute a global health emergency at this point, a WHO committee determined.
Declaring an emergency is "a major act" that can "raise anxieties," said Dr. Keiji Fukuda, the organization's assistant director-general for health security.
Despite concerns about the syndrome, researchers have not found "any increasing evidence of person-to-person transmissibility," he said.
There have been 571 confirmed cases of MERS, including 171 deaths, according to the World Health Organization. The number of countries with confirmed cases expanded to 18, with a case in the Netherlands.
Many of the cases are in Saudi Arabia and the United Arab Emirates.
Florida health care workers test negative
Two cases have been confirmed in the United States. Both patients are health care providers who were working in Saudi Arabia. The first is in Indiana; the second in Florida.
Two health care workers who came in contact with the Florida patient later went to an emergency room with flu-like symptoms. But they tested negative for MERS in a state test, the Centers for Disease Control and Prevention said Wednesday. The CDC will do its own test to confirm.
"Fortunately, this is not a virus that is spread readily in the community," Dr. William Schaffner, head of preventive medicine of Vanderbilt Medical Center said Wednesday on CNN's "New Day."
"It is spread in the context of providing health care. That's very important. And it has occasionally spread in Saudi Arabia from one family member to another. It requires close, constant, over time exposure."
People who go to a doctor or hospital with respiratory symptoms should be asked immediately whether they've traveled to the Middle East or been in contact with someone who did. "If the answer is yes, you put that person in isolation," Schaffner said. Specimens are then taken and sent to the Centers for Disease Control and Prevention for further testing. "That system is working," he said.
Representatives of 13 countries made up the WHO emergency committee that convened Tuesday through a phone conference, the WHO said.
Affected countries need to take immediate steps to improve infection prevention and control, the WHO said. The majority of infections have taken place inside hospitals.
MERS, first found in the Arabian Peninsula in 2012, is a coronavirus -- the same group of viruses as the common cold. It attacks the respiratory system. Symptoms can lead to pneumonia or kidney failure.
There is no vaccine or special treatment.
Hagel, others checked for fever
Defense Secretary Chuck Hagel got a firsthand brush Wednesday with fear of the disease in the land of its likely origin.
Before a meeting in Saudi Arabia with Crown Prince Salman Abdulaziz in Jeddah, everyone entering the room with Hagel unwittingly passed by a device screening for fever.
Its operators told reporters that they were checking for people who might be infected with the virus that causes Middle East respiratory syndrome. They found no one with a raised temperature.
WHO does not recommend such a tactic in general, Fukuda said. Some MERS patients don't have fevers, so just checking temperatures could "create a false sense of security."
The U.S. Transportation Security Administration will post CDC advisories at more than 20 U.S. airports to alert travelers about the virus.
The CDC said it will post advisories in Miami but not Fort Lauderdale or West Palm Beach.
CNN's Miriam Falco, Elizabeth Cohen, Jen Christensen and Athena Jones and the wptv.com web team contributed to this report.
™ & © 2014 Cable News Network, Inc., a Time Warner Company. All rights reserved.