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 contracted the disease before traveling through airports and the theme park, health officials said.

State health departments in California, Colorado, Utah and Washington and have confirmed cases of the extremely contagious virus, the Los Angeles Times reported on Wednesday. Taken together, the cases would account for almost 12% of the expected measles cases for the entire year (there are 220 cases per year on average, according to the Centers for Disease Control and Prevention).

The CDC calls measles the “most deadly of all childhood rash/fever illnesses”. Measles is a virus that lives in the nose and throat of those infected, and causes fever, rash, red eyes and coughing. Though there is a vaccine that is commonly given, an anti-vaccination movement has gained traction in the US despite widespread scientific criticism and debunking of the movement’s claims.

The California department of public health said on 7 January that officials believe a person infected with measles was staying in the Disneyland theme park in December. That unknown patient then infected others at the park.

Among the people exposed to measles at that time, according to the LA Times, was an unvaccinated traveler in her 20s. The woman became sick and contagious on 28 December while at the theme park. From there, the LA Times reports, she flew from Orange County to Snohomish County in Washington state. She then returned to Orange County on 3 January, and California health officials announced the outbreak on 7 January.

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.

Though an estimated 20 million people worldwide contract measles each year, the CDC only expects about 220 people in the US will become infected. American public health officials tamped down spread of the virus through the measles, mumps and rubella vaccine that children older than one year are recommended to receive two doses of, and which most college students are required to receive.

Though the vaccines are said to be 99% effective, a recent anti-vaccination movement in the US has falsely linked autism to the vaccines. Scientists have thoroughly decried the claims as false and misleading. Nevertheless, many have linked the movement to the record number of measles cases in 2014. Last year, 644 cases were confirmed, accounting for a nearly two-decade high.

This footnote was added on 19 January 2015. The California health department officials say the unvaccinated woman who was thought to have spread the disease after a visit to a Disney park in December is not the origin of the outbreak.

