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An outbreak of measles tied to a Texas megachurch where ministers have questioned vaccination has sickened at least 21 people, including a 4-month-old infant -- and it’s expected to grow, state and federal health officials said.

“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.

Sixteen people -- nine children and seven adults -- ranging in age from 4 months to 44 years had come down with the highly contagious virus in Tarrant County, Texas, as of Monday. Another five cases are part of the outbreak in nearby Denton County.

All of the cases are linked to the Eagle Mountain International Church in Newark, Texas, where a visitor who’d traveled to Indonesia became infected with measles – and then returned to the U.S., spreading it to the largely unvaccinated church community, said Russell Jones, the Texas state epidemiologist.

“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.

Infections spread to the congregation, the staff and a day care center at Eagle Mountain International.

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The ill people were all linked to the church that is a division of Kenneth Copeland Ministries. That group advocates faith-healing and advises people to “first seek the Wisdom of God” and then appropriate medical attention in matters of health, according to an online statement.

Terri Pearsons, a senior pastor of Eagle Mountain International Church and Copeland’s daughter, previously said she had concerns about 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.”

Health authorities notified the church of the first cases on Aug. 14; Texas state health officials issued a warning about the outbreak on Aug. 16. In the meantime, hundreds -- perhaps more than 1,000 -- contacts could have been affected by potentially infected people, Seward said.

“In this community, these cases so far are all in people who refused vaccination for themselves and their children,” she added.

Of the 16 cases in Tarrant County, 11 did not have any measles vaccination. The others may have had at least one measles vaccination, but they couldn’t produce documentation, county officials said.

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 virtually eradicated after a vaccine introduced in 1963. Cases now show up typically when an unvaccinated person contracts the disease abroad and spreads it upon return to the U.S.

Five previous cases in Texas this year were far higher than typical years, but don’t appear to be related to the current outbreak, state officials said.

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 -- MMR -- 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 is a senior health reporter with NBC News. Reach her on Twitter at @JoNel_Aleccia or send her an email.