Each U.S. measles eruption was sparked by an unvaccinated traveler returning from an infected region to an underimmunized pocket or school district. Experts say 2013 was a record-breaking year for measles, with 189 cases, most of them related to communities with religious objections to immunizations. That is a huge jump from 55 cases in 2012.

The resistance to getting inoculations comes from an unlikely variety of views: Deeply conservative parents reject the government’s interfering with their child rearing; orthodox religious groups dislike vaccines for being man-made, not God-given; and well-educated, health-conscious, affluent and Internet-literate parents don’t see vaccines as organic or natural.

Experts say the return of measles and, to a lesser degree, mumps is due to a decade-long backlash against common vaccines. Pockets of underimmunized children have left an unknown number of communities vulnerable to the virus.

The development of these clusters of people vulnerable to the disease has many experts worried. “We don’t have good national data, or even any data, that shows how large a threat these (unvaccinated) clusters are,” said Gregory Wallace, the CDC’s point man for measles and other infectious diseases.

Unvaccinated children and adults have been diagnosed with the sometimes fatal disease in unrelated outbreaks in states including California, New York, Oregon, Hawaii and Texas, according to the Centers for Disease Control and Prevention, or CDC.

NEW YORK — At least 53 people in 10 states have been infected with measles in the last two months, alarming officials who say the highly contagious disease was all but eradicated in the United States a decade ago.

One of the driving fears about getting their offspring vaccinated is the link that some parents have made between inoculations and autism. Jessica Plought, of Charleston, S.C., is one such parent. In August 2011, her cheerful daughter Sarah, then 2 years old, stopped talking. Soon she no longer recognized her grandparents and had no interest in books or blocks. Now 4, she is still in diapers.

Plought said her daughter’s decline followed a severe fever and rash that began with a slate of inoculation against a half-dozen diseases. She blames Sarah’s autism on the vaccines, many of which include weak strains of live viruses. “They say ‘infect to protect,’ but I don’t buy that theory,” said Plought. “They can’t tell me vaccines are safe.”

Vaccine skeptics keep in close touch with scores of often-emotional websites, Twitter feeds, Facebook pages and message boards that post research on vaccines’ harms and provide community for anguished parents. Often they are increasingly opting to delay inoculations or reject them entirely.

No one knows how many unimmunized children there are in the United States, but public health experts say the danger is growing.

“It is a problem that’s increasing — we are seeing more families express concerns about immunizations,” said Salt Lake City pediatrician Carrie Byington, vice chair of the committee on infectious diseases for the American Association of Pediatrics. The undervaccinated “are at great risk and pose a risk to others in the community.”

Some parents and doctors are particularly suspicious of the measles vaccine, which many say is linked to a gut inflammation that can cause autism. Fear of the measles vaccine and its former preservative thiomersal was stoked in 1998 when physician Andrew Wakefield published a now-discredited research paper in the prestigious British medical review The Lancet. Investigators later found that many of his subjects were referred by an insurance company trying to build a class-action lawsuit against vaccine manufacturers.

Independent studies by international medical societies, public health offices and scientific researchers have vigorously debunked his claims. Sixteen years after the Wakefield scandal, long-term studies consistently find, as does the CDC, that vaccines “are not associated with autism spectrum disorders.”

But science rarely trumps belief, especially where children are concerned.

“My mama-gut tells me I cannot do it,” said Renee Monma, a mother of two partially immunized children in Vacaville, Calif. Monma, who cites personal objections to losing control over which inoculations her children receive and when, says, “If the vaccines are so effective, then it shouldn’t matter whether my children are vaccinated.”

Parents often feel their child is protected because other children are immunized, a “herd immunity” that usually requires an 80 to 90 percent vaccination rate.

Overall U.S. immunization rates hover above the 90 percent mark. But scores of clusters nationwide are now well below that target. States with the largest proportion of health or personal exemptions: Idaho, California, Arizona, Colorado, Texas, New Mexico, Hawaii, Utah, Florida, Virginia and New Jersey.

These communities are considered particularly risky, with escalating exemption rates for whooping cough, mumps and chicken pox.

A growing number of parents say they are not against vaccines in principle but want to decide which of the immunizations to receive and to space them out over months or years to avoid overloading a young immune system.

The CDC’s mandatory vaccination schedule: 30 inoculations against 14 diseases by age 6, with boosters though elementary school. The first immunization, for hepatitis B, is administered within 24 hours of birth. Each of these diseases is highly infectious, and many can be fatal to young children or those with compromised immune systems. School districts require proof of vaccination before children can attend kindergarten.

“I don’t tell my patients not to get the vaccines,” said Ft. Collins, Colo., chiropractor Chad McMahan. “I tell them to do their own research.” Sixty percent of his young patients are not fully immunized.

None of this sits well with parents who have opted to vaccinate their own children. Many families are confident their children are safe because they have received the recommended doses. Others say they are concerned their unvaccinated neighbors pose a risk.

Alison Toro of New York City has fully immunized 2-year-old Katherine, and hopes her future classmates will also be properly innoculated.

“You hear about the backlash,” said Toro. “Scary.”

“I am in favor of personal choice, but these (diseases) are contagious, and I don’t think they know they pose a danger to other children.”