Vaccines are modern medicine's weapon against many diseases.iStockPhoto

The number of Americans requesting exemptions for vaccinations is continuing to rise, a 2012 study found. The current measles outbreak connected to Eagle Mountain International Church case in Newark, Texas, is a perfect example of how diseases that can be controlled through vaccinations are making comebacks in pockets, experts said. Before the outbreak, ministers had encouraged members to question vaccinations; they’ve changed their tune now that 21 people, including a 4-month-old baby, are sick.

Vaccination rates vary throughout the United States, in part because each state has its own requirements for exemption. Outbreaks usually occur where there are clusters of unvaccinated people, Emory University vaccine expert Saad Omer said. So even if an overall vaccination rate is high (in the Texas county home to the megachurch, for example, the overall rate is about 98 percent), a disease can spread quickly throughout a close community such as a school or church with a pocket of unvaccinated people.

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Shown is the skin of a patient after three days of measles infection. Prior to widespread immunization, measles was common in childhood, with more than 90 percent of infants and children infected by age 12.CDC

"The outbreak in Texas is the perfect example of how contagious measles is," said Dr. Paul Offit, chief of the division of infectious diseases and director of the Vaccine Education Center at Children's Hospital of Philadelphia.

The viral respiratory disease is also potentially serious: it can lead to pneumonia. Of children younger than 5 years who had it in 2011, 38 percent were hospitalized, according to the Centers for Disease Control and Prevention.

The recent situation is strikingly similar to a 2005 case in Indiana, Offit said, in which a teen went to a church picnic with 500 people after visiting an orphanage in Romania. Of the 465 people at the picnic who had been vaccinated or previously infected, three people got sick. Of the 35 who had not been vaccinated, 31 got measles.

"She was there two hours," Offit said. "You never even have to be face to face with someone; you just have to be in the same air space. It's a highly contagious disease; it can find those who are susceptible."

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Mumps is spread by droplets of saliva or mucus from the mouth, nose, or throat of an infected person, usually when the person coughs, sneezes or talks.Centers for Disease Control

Several outbreaks of mumps, another viral disease for which there is no cure, have popped up in recent years. Most of the cases have occurred in areas where people are in close contact: summer camps, college campuses, Orthodox Jewish communities.

Some of those who caught the disease were left deaf, one of the possible outcomes of the disease, Offit said. But because people rarely see the consequences of these diseases anymore, many are no longer afraid of them.

"People don't fear the disease," Offit said. "When the diseases start to come back, people are more likely to respond to their fears. We're hard-wired for fear."

Despite the risks from the actual diseases, fears have shifted to the vaccines.

"When they don't see the disease, the mental calculus shifts to concerns about the vaccine and not the disease," Omer said.

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Whooping cough is very contagious and can cause serious illness, especially in babies too young to be fully vaccinated. iStockPhoto

The number of pertussis, or whooping cough, cases hit 41,000 in 2012, the most since 1955.

While the current vaccine for whooping cough is not as effective as the previous vaccine, Offit said, it still offers eight times the protection of not having the vaccine.

Coughing fits from whooping cough can last for 10 weeks, and the disease is sometimes deadly. Because the disease is most dangerous for babies and young children, the CDC says anyone around babies needs to be vaccinated.

The Libero Ajello, CDC

Before the vaccine for Haemophilus influenzae type B existed, Hib meningitis killed 600 children a year, according to the American Academy of Pediatrics. And many who survived suffered from brain damage, seizures or deafness.

As with most of these diseases, treatments have not improved, Offit said. As many as one out of five children who survive Hib meningitis today still have brain damage or become deaf.

"You're better off preventing it than treating it," Offit said.

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Influenza (the flu) is a contagious respiratory illness caused by influenza viruses.iStockPhoto

The percentage of people needed to create herd immunity against a disease depends on the disease, Omer said. It's based on the average number of people getting secondary infections after primary infections and varies from about 80-85 percent to 90-95 percent.

"And, the herd immunity threshold assumes that everyone who refuses will be uniformly distributed," he said. "But people who refuse vaccines tend to cluster geographically. So if the five percent are clustered in one community there can be an outbreak."

A 2012 study published in Preventive Medicine shows that percentages of vaccinations that could have established herd immunity against some of the worst flu outbreaks would have ranged from 13 percent to 100 percent.

The U.S. goal of reaching 80 percent of healthy people with flu vaccine and 90 percent of high-risk people would be sufficient to establish herd immunity, the researchers concluded.