NEWARK, Texas -- Texas health officials say at least 21 cases of the measles have been linked to a megachurch in Texas.

Eagle Mountain International Church is about 20 miles north of Fort Worth, where the outbreak started when a person contracted measles overseas in Indonesia and returned to the United States.

Sixteen of those sickened by the measles cases range in age from four months to 44 years old.

"There's likely a lot more susceptible people," said Dr. Jane Seward, the deputy director for the viral diseases division at the Centers for Disease Control and Prevention.

The disease spread to the congregation, staff and day care center of the church.

"We have a pocket of people that weren't immunized," said Jones, noting that vaccination rates typically hover above the 98 percent range in his county.

Terri Pearsons, a senior pastor of Eagle Mountain International Church and Copeland's daughter, previously said she had concerns about a possible ties between early childhood vaccines and autism, a position that has been refuted by health officials.

In the wake of the measles outbreak, however, Pearsons has urged followers to get vaccinated and the church has held several vaccination clinics, according to its website. Health officials said the church administration has been very cooperative in the outbreak investigation. Pearsons did not return an email from NBC News seeking comment.

"We continue to follow up on pending and confirmed cases to help in any way we can to keep the outbreak contained," a church statement said. "We ask that others join us in prayers over this outbreak."

The outbreak raises to 159 the total number of confirmed measles cases in the U.S. this year. The disease that once killed 500 people a year in the U.S. and hospitalized 48,000 had been considered eradicated after a vaccine introduced in 1963. Cases now show up typically when an non-vaccinated person contracts the disease abroad and spreads it upon return to the U.S.

Measles is so contagious that 90 percent of people who are not immune to the disease or vaccinated against it will get sick, health officials warned. It is a respiratory disease spread by sneezing or coughing. The virus can live in the air or on infected surfaces for up to two hours. Symptoms include fever, cough, runny nose and sore throat, plus a characteristic red rash that starts on the face or hairline and spreads to the rest of the body. It can take eight days to two weeks after exposure before an infected person develops symptoms.

Health officials recommend that children receive a Measles/Mumps/Rubella vaccine at age 12 months and again at 4 to 6 years. Unless adults have previously had measles or are immunized, health officials say they should be vaccinated.

"We just want people to be aware and well-informed about the risks of the disease, especially when they travel abroad," Seward said.

JoNel Aleccia, NBC News contributed to this report.

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