The New Mexico woman was on a transatlantic flight from London to Dulles International Airport in Washington, D.C., on Feb. 20. Over the next few days, she's believed to have passed through airports in Baltimore, Denver, Albuquerque and San Diego.
New Mexico authorities refused to identify the woman, but described her as a 27-year-old who had never been vaccinated for measles as a child. Her symptoms were apparent on her flights and she was diagnosed once she arrived home in New Mexico, they said.
Most Americans are immune to measles, either because they've been vaccinated against it or are old enough to have survived it as children, before immunization shots became widespread practice. Infants who haven't yet been vaccinated are at high risk of contracting the virus, as are others with weakened immune systems.
For every 1,000 children who contract measles, one or two will die, according to the Centers for Disease Control and Prevention.
Measles starts with a fever and sniffles, and worsens often to include a reddish-brown rash on the face, tiny white spots inside the mouth and a spike in fever. It can be fatal if untreated, and while there are vaccinations, there is no cure once the virus takes hold.
People who may have been exposed to measles have a "small window" of time in which to get vaccinated or be given globulins to avoid developing symptoms, CDC spokesman Tom Skinner told CNN.
State health departments are trying to contact flight attendants and any passengers who were sitting within five rows of the infected woman on her flights last week.
Separately from the New Mexico case, public health officials in Boston held a free measles vaccination clinic on Friday after confirming a diagnosis in a 24-year-old woman who arrived in the city on a flight from France. Two people in Boston are suspected of since having contracted the disease, CNN reported.
More than 10 million people are infected with measles worldwide each year, and the disease is the leading cause of vaccine-preventable deaths in small children. Outbreaks are more common in Europe than in the United States, and most U.S. cases come from transatlantic travelers. U.S. law requires that any cases of measles be reported to public health authorities.
"We don't want measles to be imported back into the U.S. once it gets a foothold," Dr. William Schaffner, an infectious disease specialist at the Vanderbilt University School of Medicine in Nashville, told The Associated Press.