(NBC News) — More than 50 people have been diagnosed with the measles in an outbreak that began in December at California’s Disneyland.
“We are very concerned, and it’s expanded much more rapidly than we thought,” said pediatrician Dr. Robert Bjork.
Cases have now been reported in Utah, Colorado and Washington state.
The airborne virus is so contagious it can stick around after an infected person has left the room.
“The virus can actually survive on dry surfaces for hours, and that’s what’s most concerning about this,” said Dr. Natalie Azar with NYU’s Langone Medical Center.
Measles causes fever, a cough and runny nose. It’s also marked by a signature rash that may not show up until after the patient has been contagious for days.
“It can lead to blindness, it can lead to encephalitis, which is an infection of the brain,” said pediatrician Dr. Kjartan Armann.
Thirteen people are also sick in a separate measles outbreak in South Dakota.
One clinic there offered booster shots for free.
“You’re not just putting yourself at risk, it’s others you’re putting at risk, too, if you get sick and pass it along,” said Eric Van Meter, a South Dakota resident.
Many patients in the outbreak that began in Disneyland were not vaccinated, either because they were too young or because parents decided against vaccinating their children. Doctors say a decrease in vaccination rates is the main reason the nation has seen a marked increase in measles outbreaks in recent years.
More than 600 people were infected in the U.S. in 2014, the worst year for measles in over a decade.
Doctors say those born in the 1970s or ’80s may need a second round of shots for full immunity because they may not have received a booster shot that was later added to the vaccine schedule.