( CNN ) -- The spread of the potentially fatal Middle East respiratory syndrome has become more serious and urgent , the World Health Organization said Wednesday . At the same time , WHO said that -- for now , at least -- the illness known as MERS does not constitute a global health emergency . 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 , WHO reported Wednesday . Many of the cases are in Saudi Arabia and the United Arab Emirates . Even without any official worldwide alert , Anne Schuchat , the head of the CDC 's National Center for Immunization and Respiratory Diseases , acknowledges that `` this is a relatively new virus that does have a high fatality rate , '' ample reason to pay attention . Authorities have n't pinned down all the details about how exactly it arose and how it spreads , though Schuchat said , `` we do n't have evidence right now that this is airborne ... the way the measles virus is . '' `` We do n't know as much as we would like so far , '' the CDC official and assistant surgeon general in the U.S. Public Health Service told CNN 's Brooke Baldwin on Wednesday . `` ... But we 're tracking it and trying to understand . '' 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 CDC said Wednesday . The Florida MERS patient , specifically , was fever-free and continuing to improve Wednesday while undergoing treatment at Orlando 's Dr. P. Phillips Hospital , the state 's health department said . Given the virus ' apparent roots in the Arabian Peninsula , Schuchat said it 's important that health care workers ask anyone who has suspicious `` respiratory symptoms '' where they have been in recent weeks . `` More and more people are going to be traveling the next couple of months , so we really want clinicians to be on the lookout , '' she said . Commercial airlines are on guard as well , including efforts to make sure those who might have come into contact with the two MERS patients in the United States did n't come down with the virus . American Airlines said in a statement that it is `` working very closely with the Centers for Disease Control , and contacted our flight crew as soon as we learned of ( one such ) diagnosis . The CDC is in the process of contacting passengers to advise them of any necessary precautions . '' `` We are leaving no stone unturned in ensuring the good health of our employees and customers , '' the airline said . And Delta Airlines said that , like all carriers , it `` has a long-standing protocol for working with the U.S. Centers for Disease Control , as we are in this instance . The safety of our customers and employees is Delta 's top priority . '' One traveler who was on one possibly affected flight told CNN affiliate WMKG that the call she got about one confirmed case of MERS left her `` really scared . '' Twelve days after her flight , she said , her state health department called and sent her a note asking whether she had symptoms such as fever or cough , and also saying she should record her temperature for two weeks . `` I was in shock that I could actually contract it , '' she said . The woman , who did not want to be identified , said she and her husband so far had no symptoms . Another traveler , Ben Kinney , told CNN affiliate WXIA that the CDC called him twice in two days , saying he was on a flight from Boston to Atlanta with someone who had MERS . He said officials would not disclose how close he was to the patient . Four people -- each from different families -- have been tested in San Antonio for MERS because they were on a plane with one of the two confirmed U.S. patients , said San Antonio health department spokeswoman Carol Schliesinger . None of those four had any suspicious symptoms , she added . Tests for two of them have come back negative for MERS , while the other results are expected in the next couple of days . MERS does n't spread like flu . 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 . Besides those who contract the virus by coming into `` very close contact '' with someone suffering , Schuchat cited `` a lot of interesting studies ... about camels and whether some sort of exposure to camels or their products might be important '' in explaining MERS origins . While little is known definitively , `` Fortunately , so far , it has n't shown itself easily to spread from person to person , '' she added . Dr. William Schaffner , head of preventive medicine of Vanderbilt Medical Center explained Wednesday on CNN 's `` New Day '' that MERS can `` 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 CDC for further testing . `` That system is working , '' he said . Florida 's health secretary , Dr. John Armstrong , said Wednesday that the case in his state `` is contained . '' `` There is no broad threat to the general public , '' Armstrong said . 5 things to know about MERS . Hagel , others checked for fever . 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 . 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 do n't have fevers , so just checking temperatures could `` create a false sense of security , '' he said . The CDC is not recommending that anyone change travel plans . The U.S. State Department has instructed its embassies and consulates in Saudi Arabia , Oman , Qatar , Jordan , Kuwait , and the UAE to provide U.S. citizens with CDC 's general guidance for infection control , a State Department official said . CNN 's Greg Botelho , Miriam Falco , Elizabeth Cohen , Debra Goldschmidt , Aaron Cooper , Jen Christensen and Athena Jones contributed to this report .
NEW : Spokeswoman : Two tested in San Antonio for MERS ; none has symptoms . A case in the U.S. brings the total to 4 countries affected . CDC official : `` We do n't know as much as we 'd like so far '' 18 San Antonio airlines are coordinating with health officials over cases in U.S .