A North Texas measles outbreak has health officials urging residents to get vaccinated against the highly contagious illness in time for the first day of school.
Nine people have come down with the measles over the last month in Tarrant County, county health department spokesman Al Roy told ABCNews.com. One person caught measles traveling outside the United States, and the other eight adults and children caught it from that first person, he said.
"There are potentially hundreds of people exposed in the North Texas area," state Health Commissioner Dr. David Lakey said in the health alert that went out Friday.
Although the MMR vaccine should offer lifelong protection against the measles, Texas is a state that allows children to attend school without it if their parents fill out personal belief exemption paperwork. Children can be exempt from entering school without the required vaccinations "for reasons of conscience, including a religious belief," according to the state health department.
"Measles is probably the most contagious virus that we know of," said Dr. William Schaffner, chairman of preventive medicine at Vanderbilt University Medical Center in Nashville, Tenn. "More importantly, there are an increasing number of children whose parents are delaying immunization or stretching them out, and others who are simply withholding these children from immunizations because of autism."
Schaffner said the autism argument "has been laid to rest scientifically," but parents still believe it and don't vaccinate their children.
Almost 92 percent of Texans received MMR vaccinations, making its estimated vaccination rate better than 30 other states, according to 2010 data from the Centers for Disease Control and Prevention, which is the most recent data available.
Schaffner said measles is so contagious, however, that even a small number of unvaccinated people won't be protected by the so-called herd effect if someone brings the measles virus to the community.
Measles causes flu-like symptoms and a rash, according to the CDC.
Complications include pneumonia and encephalitis, so several hundred children used to die from it every year before MMR vaccines were required in 1957, Shaffner said. As a result, he said it's a problem when people don't get vaccinated for it.
"We're talking about numbers of cases that can be counted on two hands," he said of the current outbreak. "We think of each of these as a major public health failure today, and it's very, very sad."
Although Tarrant County officials are worried about adults and children who are not vaccinated -- especially those who are somehow against getting vaccinations -- they are glad this is happening now because back-to-school vaccination clinics and programs are already under way.
"It could have come at a worse time but it didn't," Roy said.
Five other cases had been reported elsewhere in Texas earlier in 2013, but those aren't considered connected to this outbreak, according to Christine Mann, a spokeswoman for the Texas Department of State Health Services.