"The fact that we have lower immunization rates makes the possibility of an outbreak like this - getting so out of control that it circulates for a long time with a lot of cases - more likely and makes it harder for us to quickly control it," said Dr. Bob England, director of the Maricopa County Department of Public Health.
England's comment followed his agency's announcement Wednesday that an unvaccinated person returning from Europe tested positive for measles after visiting several crowded places in the Valley in late March, likely exposing others.
"In this case, even with relatively high immunization rates, so many people were exposed we think it's likely we're going to see more cases pop up," England said.
"The problem with measles in particular is it is widely infectious. It is the most infectious disease known to man," he said.
Cases of diseases like measles have become rare because of high rates of vaccinations that result in a herd immunity, a level of immunity in a population that makes a disease unlikely to spread, he said.
"So when one person's disease comes into the community like this, it finds a hard time finding another person to jump to," England said.
But in recent years the rates of Arizonans choosing to exempt their children from vaccines, whether for medical or personal reasons, has increased. According to Rep. Debbie McCune Davis, D-Phoenix, program director of The Arizona Partnership for Immunization, kindergarten exemptions have increased statewide from 1.8 percent in 2003 to 4.2 percent in 2013.
"It has been a gradual increase, and the bulk of them are personal exemptions, not medical exemptions," she said.
Among kindergartners statewide, 5.5 percent during the 2012-2013 school year weren't immunized against measles, mumps and rubella, according to the Arizona Department of Health Services.
McCune Davis said researchers at the University of Arizona have found that convenience and parent refusal were the main reasons for exemptions.
"There's a couple of reasons for that," she said. "One of them is diminished perception of risk of disease. Simply put, our young parents don't see these diseases any more so they don't feel threatened by them."
The second reason has to do with parents' concerns about the safety of vaccines, she said.
Jessica Rigler, bureau chief for epidemiology at the state Department of Health Services, said her department is also concerned about the dropping vaccination rates.
One reason for concern is the intensity of measles, she said.
"Ninety percent of unvaccinated people that come into contact with a measles patient are expected to contract the disease," Rigler said.
In addition, the virus can survive for hours after the infected individual leaves the room, she said.
For individuals who may be too young or ill to get vaccinated, Rigler said they depend on herd immunity.
"It's very important that other members that are able to be vaccinated are, because that will help to keep the whole community safe," she said.
According to England, if immunization rates continue to trickle down, outbreaks of preventable diseases will happen more often.
"Nothing screams that we're all in this together quite like vaccines do," he said. "What I'm doing by getting vaccinated isn't just helping me, it's helping to make sure I'm not a vector of disease to someone else."
He added: "What worries me the most is we're going to have to learn the hard way by seeing lots of people get sick and some people get seriously ill before we realize that this is real, this is serious. And we have to return to requiring vaccines for school and so forth."
In this particular case, England estimated that as least several hundred people were exposed to the disease. By this point, infected people could be showing signs of the illness, he said.
"We hope that by alerting people and getting them diagnosed early with that first generation of transmission that may have happened, that we can then find them and identify their close contacts and cut it off before it goes any further," he said.