At least 21 people, including a 4-month-old infant, have been struck by measles in an outbreak traced to a Texas megachurch that belongs to a sect that advocates faith-healing, and where a pastor had previously questioned the safety of vaccines.

Health officials think a visitor to the Eagle Mountain International Church carried the virus that causes measles back from a trip to Indonesia. Measles then spread throughout the church’s congregation, staff and a day care center. Pastor Terri Pearsons, who previously voiced concerns about alleged ties between childhood vaccines and autism, is now urging her congregation to get vaccinated, according to NBC News.

Before the measles vaccine was introduced in 1963, the disease claimed about 500 lives a year in the U.S. alone. The disease is extremely contagious, and spreads easily through coughs and sneezes. Most patients will recover, but some will become susceptible to other diseases, like pneumonia, and occasionally there are fatal cases.

Vaccines are primarily effective at eradicated illnesses by bestowing “herd immunity,” where so much of a community is protected against a disease that there is a firewall against the spread of a contagion. The more people that are vaccinated against an illness, the less likely it is that an infected person could come into contact with someone vulnerable and thereby touch off a chain of infections. And while medical studies have found no credible proof of a link between vaccines and autism, many parents still fear a cluster of vaccinations early in life can harm their children.

When large groups of people start to go unvaccinated, that provides new avenues for an illness to circulate. This can be especially dangerous for certain vulnerable subpopulations, like very young babies, who can’t be vaccinated until they reach a certain age.

Here’s a roundup of recent disease outbreaks that have been tied to pockets of unvaccinated citizens:

November 2012-July 2013, Wales, U.K.: More than 1,200 people came down with measles in this outbreak, and 88 patients had to be hospitalized. One 25-year-old man died after contracting pneumonia as a result of measles. In August, officials said that up to 30,000 Welsh children were at risk of catching the virus, according to Wales Online. Some have linked the outbreak to a “missing generation” of people that did not get the measles-mumps-rubella (MMR) vaccine in the 1990s as children. At the time, the South Wales Evening Post ran a series of articles questioning the safety of vaccines, though the paper did not go so far as advocating for parents not to immunize their children.

March 2013, Brooklyn, N.Y.: Dozens of measles cases arose in several Orthodox Jewish communities. The outbreak sent two pregnant women to the hospital (one of the women miscarried). New York City health officials traced the origin of the outbreak to a person that brought the virus back from a trip to London. While most Orthodox Jewish communities in Brooklyn have high vaccination rates, there are small pockets of families that choose to go unvaccinated – and the outbreak started in these pockets.

1990, multiple states: Rubella (German measles) reared its head again in the U.S. in the 1990s, with more than 1,000 people sickened, according to the Associated Press. Many of the cases occurred in adults, and large clusters of patients belonged to religious communities like the Amish that eschew vaccination.

Still, high vaccination rates amid the general population are keeping the the horrific outbreaks of centuries past at bay. Even with the Texas outbreak, there are still only 159 total confirmed measles cases in the U.S. thus far in 2013.