More than two dozen cases of measles have now been linked to Disney theme parks in Southern California.
The California Department of Public Health reported seven more cases on Monday, bringing the total number to 26 people in four states.
Health officials say at least 8 of those infected had not been vaccinated.
Most of the patients visited Disneyland or Disney California Adventure between Dec. 15 and Dec. 20, but some may have contracted the illness from others who were at the parks.
Officials say 22 of the cases are in California, two are siblings in Utah, and there is one case each in Colorado and Washington. The patients range in age from 8 months to 21 years old.
"This could easily turn from two people into dozens, hundreds, if you don't jump on it very quickly," Dr. Joseph Miner, executive director of the Utah County Health Department, said of the cases in his state. CBS Salt Lake City affiliate KUTV reported the Utah patients were children who visited Disneyland the week before Christmas.
Measles is a highly contagious virus that can be spread through coughing or sneezing. Infected droplets may remain contagious on surfaces for several hours. People can spread the disease before they even realize they have it.
"The real difficult thing with measles is you become infectious four days before the rash starts, so even before you recognize it as measles, you can be infecting other people," Dr. Helene Calvet, Orange County deputy health officer, told CBS News.
The Centers for Disease Control and Prevention says symptoms of measles typically appear one to two weeks after exposure and may include runny nose, watery eyes, cough and a high fever. Then a telltale raised, red rash starts to spread on the face and down the body. The rash usually lasts four to seven days.
Nationwide, the measles vaccination rate is over 90 percent, and most outbreaks begin when someone who wasn't vaccinated contracted the virus it overseas. Calvet noted that a crowded place full of international visitors like Disneyland would be an easy place for it to spread.
"People who believe that measles is no longer a threat, well I think this outbreak and the outbreak from last year are perfect examples that measles can still circulate in California," she told CBS News chief medical correspondent Dr. Jon LaPook.
California had its worst outbreak in decades last year, and many cases were linked to families who opted out of having their children vaccinated. Overall the U.S. had 644 confirmed cases of measles in 2014, the most in 20 years.