What started as a measles outbreak among seven people who visited Disneyland in December has spread to more than 26, as an unvaccinated California woman apparently transmitted the virus through airports and the theme park, health officials said. The California Department of Public Health confirmed January 7 that the outbreak could be traced back to a group of people who had visited either Disneyland or Disney California Adventure Park (both in Orange County, California) over a specific period in December. The parks are popular tourist spots that provide ripe conditions for the spread of highly infectious diseases, with hordes of people waiting in lengthy lines that often snake into cramped spaces.

That level of infectiousness is particularly concerning considering how many children visit Disneyland who are too young to be vaccinated. Two of the Disneyland cases were too young to be vaccinated – the first dose is recommended at 12 months – so those parents never had the choice to protect their children from measles. Other parents’ decisions not to vaccinate their children made that choice for them. “We are working with the health department to provide any information and assistance we can,” Dr. Pamela Hymel, chief medical officer for Walt Disney Parks and Resorts, said in an emailed statement.

The virus is highly contagious, can live for up to two hours on surfaces and is transmitted through an infected person’s coughs or sneezes. Measles is so contagious that “90% of the people close to that person who are not immune will also become infected”, according to the CDC.

Last year saw the largest number of measles cases in the U.S. since it was eliminated in 2000, according to the Centers for Disease Control and Prevention (CDC); there were 644 cases and 23 outbreaks across 27 states, a spike from 2013, when there were fewer than 200 cases. Before the measles vaccination program began in the U.S. in 1963, 3 million to 4 million people contracted the disease each year, which killed 400 to 500 annually.

The majority of people who get measles aren’t vaccinated and measles can spread when it reaches a community where groups of people are unvaccinated, the CDC says. Measles is spread through water droplets that travel through the air from coughing or sneezing.