MERS has similarities to Severe Acute Respiratory Syndrome, or SARS, which broke out in 2003-2004 in Asia and eventually spread to over two dozen countries. Both diseases are caused by a coronavirus, so named for the crown-like spikes on its surface. Their symptoms aren't easily distinguished from other respiratory illnesses: fever, cough, shortness of breath. Both are virulent; SARS had an overall mortality rate of 15 percent, and greater than 50 percent in those aged 65 and older. MERS, which was identified in the Middle East in 2012, so far has an estimated mortality rate of 30 percent. Of around 400 known cases of MERS to date, over 100 people have died. But this number may change as more cases are identified, particularly those where there are no visible symptoms.