The California outbreak likely started when a traveler who was infected overseas with measles visited the amusement park while infectious, according to the Centers for Disease Control and Prevention.
But health officials don't know exactly who the source of the outbreak is.
Genetic analysis of
the specimens from 30 California patients showed that
the measles was of genotype B3, which is identical to the virus circulating in the Philippines. The CDC also cautioned that the same virus type has been found in 14 other countries.
The highly contagious disease has been damaging in the Philippines, infecting about 53,000 people and killing 110 people in 2014. The country has not seen outbreaks this year, although there have been a trickle of cases, said Dr. Julie Lyn Hall, the WHO Country Representative in the Philippines.
So far, the United States has 141 reported cases of measles
this year -- 98 of them from California. Most of the people who've become sick with measles were unvaccinated, according to the CDC.
Prior Philippines links
This is not the first time that measles affecting the Philippines has been linked to U.S. outbreaks.
The virus can easily travel internationally -- as was the case of the unvaccinated Amish missionaries who brought back measles from the Philippines. That outbreak in Ohio infected 383 people in 2014.
During the same year, 25 U.S. travelers, most of them unvaccinated, became ill with the measles after returning from the Philippines, according to the CDC.
Measles was declared eliminated in the United States in 2000, which means it is no longer native to the country, but continues to be brought in by international travelers.
The United States gets imported cases every year, according to the CDC.
Visitors may pick up the disease and bring it back to the U.S., potentially infecting infants who cannot be vaccinated because they are too young, or people who intentionally remain unvaccinated.
Disney parks are a popular destination for international visitors, with as many as 24 million patrons a year. And it becomes especially busier during the holiday season. The first measles case linked to Disneyland was reported on January 5, in a case involving an 11-year-old unvaccinated child who visited the park, according to the California Department of Public Health.
The current outbreak has triggered a fierce debate about parents who choose not to vaccinate their children.
Why the outbreak in Philippines has been so serious
After Typhoon Haiyan struck the Philippines in November 2013, the country grappled with a massive measles outbreak.
In 2013, the country reported 5,799 confirmed cases of the disease. By 2014, that number increased nearly tenfolds to 53,357 cases, according to World Health Organization data. The spread of the virus
has been exacerbated by mass migration, with nearly 4 million displaced people.
"Many of those [measles] cases were associated with Typhoon Haiyan as a number of people were moving out of the typhoon-affected area and coming into Manila," said Hall of WHO.
They arrived into places with low levels of measles vaccine coverage in Manila, triggering a severe outbreak in January and February 2014.
The number of cases that year was unusual compared to previous ones, Hall said.
The WHO and the Philippines Department of Health conducted vaccination campaigns to immunize 11 million children.
"There's still the measles virus in the Philippines," Hall said. "Despite all the efforts, not all children are protected from measles here. We are still getting reports, but nowhere near the level of the same time last year."