California health officials working to track down the origins of a measles outbreak are reportedly looking to the Philippines, as the number of cases in the U.S. continues to rise.

Researchers have determined that the virus shares the same genetic material as the type most commonly found in the Philippines, the LA Times reported. More than 50,000 were sickened by the virus in the Philippines last year, with 110 cases resulting in death.

While officials said it is likely that the virus originated in the Philippines, they are still working to determine how it traveled to California’s Disneyland, the LA Times reported.  The number of cases in the U.S. has risen to 141, and officials say most of the people who have contracted the virus were not vaccinated.

Twenty new cases were added Tuesday to the tally by the Centers for Disease Control and Prevention: 10 in California from the Disneyland outbreak, 8 from the suburban Chicago outbreak at a day care center, and two unrelated cases in Nevada. Cases have been reported in 17 states and the District of Columbia.

The Philippines has been linked to measles in the U.S. previously, with at least two other instances in 2014 involving an Amish humanitarian aid worker who caused an outbreak in Ohio, the LA Times reported. More than 380 people in Ohio were sickened as a result of the outbreak.

Scientists noted that the type of measles found in the Philippines has also been traced to 14 other countries, including England, Germany and six American states, according to the LA Times. The results have been reported to state officials. They have not been able to pinpoint a visitor or family behind the outbreak, but know that they visited or worked at the theme park between Dec. 17 and Dec. 20.

“The genotype is really the genetic fingerprint of the virus, and so it certainly tells you the number of cases that we have been seeing in California likely all relate to that individual that began at Disneyland,” Dr. Matt Zahn, medical director for Orange County told the LA Times.

Before a measles vaccine became available 50 years ago, most children contracted the highly contagious illness by their 15th birthday. Measles was declared eliminated in the U.S. in 2000, and until recently, it was unusual to see more than 100 measles cases a year. Those cases are usually brought in and spread by travelers from countries where the illness is more common.

Measles is usually seen in winter and early spring. It causes a fever, runny nose, cough and rash all over the body. In rare cases, it can be deadly.

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The Associated Press contributed to this report.