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.

"It's a shock to younger physicians that their own waiting room or emergency room is where people can get measles," said the CDC's Dr. Anne Schuchat.

Also on Thursday, a medical journal - the Annals of Internal Medicine - released a commentary warning doctors to prevent that kind of situation.

"We must ensure that our facilities do not become centers for secondary measles transmission," wrote Dr. Julia Shaklee Sammons, an infectious disease specialist at the Children's Hospital of Philadelphia.

She urged doctors who suspect a measles case to place the patient in an isolation room with special ventilation that keeps the air from circulating around the building. Doctors and nurses should also wear surgical masks or respirators to protect themselves from getting infected, and to ask the infected patient to wear a surgical mask too.

The measles virus spreads easily through the air, and in closed rooms. Infected droplets can linger for up to two hours after the sick person leaves.

It causes a fever, runny nose, cough and a rash all over the body. In rare cases, measles can be deadly, and is particularly dangerous for children. Infection can also cause pregnant women to have a miscarriage or premature birth.

Before a vaccine became available about 50 years ago, nearly all children got measles by their 15th birthday. In those days, nearly 500 Americans died from measles each year.

A bad resurgence of measles hit the nation in 1989 to 1991, when 55,000 cases were reported. That flood of cases was blamed on a widespread failure to vaccinate uninsured children.

In reaction, the federal government started a program in 1994 to pay for vaccines for kids who are uninsured, in the Medicaid program, or meet other criteria.

In a report released Thursday, CDC officials estimated that the program and other childhood vaccination efforts will prevent 322 million illnesses and 732,000 premature deaths over the course of the lifetimes of children born in the years 1994 through 2013.

Wrapped into that estimate are 71 million measles cases, nearly 9 million measles hospitalizations and 57,000 measles deaths.

There has been no measles deaths reported in the U.S. since 2003. "But the way we're going, we feel it (another) is inevitable," Schuchat said.

Today, the measles vaccination rate is above 95 percent for children of kindergarten age. But there has been a small but growing trend of parents seeking exemptions for their children from school-entry vaccination requirements for religious or philosophical reasons. Other parents have tried to space out, or delay, measles vaccinations because of fears that the shot will trigger autism or other problems.

Measles outbreaks are more common in communities where such beliefs are more common, experts say. "It's often concentrated there. Folks who think similarly tend to live in the same neighborhood or attend the same religious organization," Schaffner said.

The CDC national tally may already be outdated. Also on Thursday, Ohio officials announced a new cluster of 13 suspected cases in rural Knox County. At least three traveled to the Philippines last month on a religious humanitarian mission.

About 17 percent of U.S. cases this year were vaccinated. Health officials say that although the vaccine is very effective, it's not perfect.