Health officials are worried about recent U.S. measles outbreaks that so far have caused more illnesses than at the same point of any year since 1996.

Authorities say 129 cases in 13 states were reported by mid-April, the bulk of them in California and New York City. Most were triggered by travelers who caught the virus abroad and spread it in the United States among unvaccinated people. Many of the travelers had been to the Philippines, where a recent measles epidemic has caused at least 20,000 illnesses.

The U.S. numbers remain relatively tiny, but officials are worried to see case counts growing. Since 2000, the highly contagious disease has been considered eliminated in the United States, aside from occasional small outbreaks sparked by overseas travelers. For most of the last decade, the nation was seeing only about 60 cases a year.

But since 2010, the average has been nearly 160.

"This increase in cases may be a 'new normal,' unfortunately," said Dr. William Schaffner, an infectious disease expert at Vanderbilt University in Nashville.

Contributing to the problem: Decades of measles vaccination campaigns have been so successful that many doctors have never seen a case, don't realize how contagious it is, and may not take necessary steps to stop it from spreading.

Among the 58 cases reported from California, at least 11 were infected in doctor's offices, hospitals or other health-care settings, according to a report released Thursday by the Centers for Disease Control and Prevention. New York City health officials say two of their 26 cases were infected in medical facilities.

-- ASSOCIATED PRESS