The Centers for Disease Control and Prevention on Thursday reminded doctors to ask patients with severe respiratory illness if they recently travelled.
Officials are now urging testing for MERS in very sick patients who had visited a South Korea hospital within two weeks of falling ill.
MERS was first detected in 2012 in the Middle East. More than 1,200 cases have been confirmed, including two travellers to the United States last year.
The MERS virus is thought to spread through sneezing and coughing, but not as easily as the flu or some other diseases.
CDC MERS page: http://www.cdc.gov/coronavirus/mers/index.html