Wendy and Gerry Waldron have a jumbo-size jug of Purell at their front door. Visitors can’t as much as set foot inside the family’s home until they rub the sanitizing liquid onto their hands, and those with mere coughs and sniffles are denied entry.

The Mentor, Ohio, family isn’t germophobic. Or at least they weren’t until Aug. 28, 2014— when their 5-year-old son, Liam, was diagnosed with osteosarcoma, a rare cancer of the bone that is most common in teenagers and children. He is on his fourth cycle of chemotherapy, a treatment that kills the cancer cells but also his infection-fighting white blood cells.

Gerry and Wendy are among a handful of parents in the United States who are on the defensive against germs— not only due to their son’s life-threatening cancer but also because of another, vaccine-preventable illness that poses a risk to his life: measles.

“No kids come over; he goes to no one’s house,” Wendy, 46, an event manager at a music institute in Cleveland, told FoxNews.com. “He wants to go out and play in the snow, he wants to build a snowman, he wants to go to Hong Kong and Rio de Janiero, and hopefully when we’re done with this and he beats it, he can.”

The threat to cancer patients

Since December, the Centers for Disease Control and Prevention (CDC) has reported over 120 measles cases in the U.S., a record number in recent history for this early in a calendar year. Measles was thought to be eliminated in the nation in 2000, but travelers coming to and from foreign countries have infected unvaccinated children, spurring an outbreak in 2014 that led to more than 600 U.S. measles cases.

This year and last, many of the cases have occurred in children whose parents opted out of vaccination based on personal belief.

“The failure of other parents to safely vaccinate their children against these diseases puts Liam and these other children at risk— and that risk includes death,”  Liam’s doctor Margaret Thompson, a staff physician in the department of pediatric hematology, oncology, and blood and marrow transplantation at the Cleveland Clinic, told FoxNews.com.

Liam had been up to date on his vaccines before, but following his diagnosis, Wendy and Gerry, 50, a director in product at a contract electronics manufacturer, couldn’t move forward with the rest of the CDC’s recommended vaccine schedule, which would have been administered when Liam was 5 years old.

The MMR vaccine— a shot that provides immunity from measles, mumps and rubella— contains a live, highly attenuated virus that exposes the immune system to a degree of the infection and trains the body to fight potential future encounters with the more robust, actual virus. In healthy kids, the exposure to the attenuated virus creates said immune response, but suppressed immune systems— like Liam’s— don’t have the strength necessary to prevent the virus from overwhelming their bodies.

“The baby’s immune system is vast and extremely capable of handling so many germs in the world,” Patty Stinchfield, a pediatric nurse practitioner in infectious disease at Children’s Hospitals and Clinics of Minnesota, told FoxNews.com. “If the baby’s immune system wasn’t competent, they wouldn’t survive. But we have some rare immune deficiencies … and those are the kids that would be vulnerable to getting vaccines.”

Stinchfield, 57, also a director of infection prevention and control at Children’s, guides cancer patients and their caregivers through treatment, and educates them on protective measures in the process.

She helped Laura Bredesen, of Independence, Minn., navigate her son Ben’s cancer treatment. Ben, who is 7, was diagnosed with acute lymphoblastic leukemia (ALL) when he was 2 and a half years old, and he finished his chemotherapy treatment in August 2013. ALL is a cancer of the blood and bone marrow that is common in children.

“It definitely made me a permanent germophobe,” Bredesen,  37, a part-time elementary school substitute teacher, said of her son’s diagnosis. “His immune system was so compromised that there was a fear of, ‘Is there mold in the sandbox that he likes to play in?’ or ‘Is there a child over that doesn’t know he or she is sick, and could give Ben a cold that could cause an infection?’”

In most people, measles will cause cold-like symptoms, as well as light sensitivity, and a raised, red rash that starts at the head and spreads downward and across the entire body. In one in 20 cases, measles gets into the throat, leading to pneumonia and breathing problems that often require hospitalization. About one in 1,000 children will get measles in their brain, which may result in permanent damage to the organ as well as deafness. Statistically, the same number of children will die.

For youth with suppressed immune systems, catching measles could mean delayed cancer treatment or death.

Measles is a highly contagious virus— even more contagious than influenza, Stinchfield said. Measles molecules are smaller and more lightweight than the flu’s, and they stay suspended in the air, whereas flu molecules fall to the floor. To infect other people, someone with measles doesn’t even have to cough or sneeze. According to the CDC, if a person with measles enters a room, nine out of 10 of the other unvaccinated people in the space will become infected.

In March 2011, Ben had a close call when doctors realized an unvaccinated child was in the same hospital unit he stayed in for a routine spinal tap.

Ben received immunoglobulin shots and went into isolation for 21 days as a precaution. At the end of his quarantine, doctors confirmed that Ben did not have measles.

“It was really unthinkable to be dealing with your child trying to fight for his life,” said Bredesen, who wrote about the scare on a pro-vaccine blog, Voices for Vaccines. “Against leukemia is one thing, but then on top of that, it was unfathomable to think then that he would be exposed to a vaccine-preventable illness, which would be life-threatening to him. It was very unbelievable, and I’m unable to fully understand.”

Today, Ben is healthy and fully vaccinated.

Why some parents don’t vaccinate

According to the CDC, about 95 percent of U.S. kindergarteners have received both doses of the MMR vaccine, the first of which is administered at 12 months and the second between ages 4 and 6. The other 5 percent opts out for various reasons, most commonly on a philosophical or religious basis. Twenty states allow philosophical exemptions from vaccines, and— except for West Virginia and Mississippi— all states and the District of Columbia allow religious exemptions. Medical exemptions, such as for Liam, the osteosarcoma patient, are rare but allowed in every state.

Other parents delay vaccination or follow a self-styled vaccination schedule, and decide against following age-based vaccination recommendations from the CDC, which include over 20 doses to be administered from infancy to age 19.

Those that opt against vaccinating their kids may still hold stock in a since-retracted study published in 1998 in The Lancet that suggested a link between vaccines and autism. The lead author of that study, Dr. Andrew Wakefield, reportedly altered the medical histories of his study participants to falsify data. He has since been stripped of his medical license in Britain.

Some parents, like Mary Ann Block, 69, a physician in Hurst, Tex., believe that the link may still hold true, that the measles aren’t that serious—“I had measles. My mother had measles. We all had measles,” she said— and that, in general, many medicines the government recommends simply aren’t necessary.

“The thing about it is the people who want to vaccinate should be able to vaccinate,” Block told FoxNews.com, “but the people who don’t want to vaccinate should be allowed to do that too.”

Block, who received her D.O. from the University of North Texas Health Science Center— then known as the Texas College of Osteopathic Medicine— in 1989, has focused on using holistic treatments for chronic health conditions in her practice for the past 25 years. She said she fully vaccinated her daughter but now regrets it.

A 2011 study published in the journal Public Health Reports revealed the majority of parents who opt out of vaccinating their children do so because they feel there isn’t enough research to suggest their benefits outweigh their possible side effects.

Tyson Perez, 39, a pediatric chiropractor in the Carlsbad, Calif., area, falls into that group. He and his wife have both been fully vaccinated, but when they had their daughter, Cambria, who is 2 and a half, they began researching vaccines and opted out on the basis of philosophical belief.

“I weighed the absolute risk of the vaccine versus the hypothetical risk of the illness, and it became clear to me that the absolute risk of the vaccine outweighed the hypothetical risk of the illness,” Perez told FoxNews.com.

Like Block, Perez approaches medicine holistically. His daughter has never had any medicine, vaccine, or antibiotic— not even Tylenol for a fever.

“We make sure her neurological system is optimal— optimal nutrition, lots of love, good sleep, lots of sunlight, and all of the things that our great-grandparents used to use,” Perez said. “And the benefit to that is when you have a strong immune system, you can fight off anything that’s out there.”

Perez said he was aware that measles poses a health risk to children with immunosuppressive diseases, but he argued that immunoglobulin treatments in the event of a potential exposure and quarantine during cancer treatment are appropriate preventative measures.

“Aside from bolstering their immune system, it’s keeping them [immunosuppressed children] separated from the general population,” Perez said.

When immunity isn’t possible

Paul Offit, director of the Vaccine Education Center at the Children's Hospital of Philadelphia, and a pediatrics professor at the University of Pennsylvania, has spoken to various media over the past decade, commenting on vaccination trends and arguing against the allowance of non-medical exemptions. Like nearly all scientists who have conducted peer-reviewed research on vaccines, he says vaccines are the only way to prevent outbreaks of infectious diseases like measles.

He said that fears over vaccines, namely that vaccines themselves cause outbreaks because they're not attenuated enough, aren’t warranted.

“When you’re infected with [the] natural measles virus, the virus will reproduce itself thousands of times in your body. If you’re inoculated with the measles vaccine in your body, it reproduces only about 20 times, and not enough to spread [to others]— just enough to create an immune response,” Offit, also a co-creator of a rotavirus vaccine, told FoxNews.com.

According to the National Institutes of Health (NIH), an attenuated vaccine is one that includes a virus' microbes that are manipulated so they become weaker when injected into human beings.

Offit said it is safe to administer the measles, mumps, rubella, chicken pox and rotavirus vaccines in the homes of people who are immunocompromised because these live, attenuated vaccines are so weak that they can’t spread through person-to-person transmission or through the vaccine vials themselves.

Offit added that, contrary to some parents' belief, the only polio vaccine offered in the U.S. is inactive. The active, live version is still used in other parts of the world, and it runs the risk of the virus reverting to its wild form and leading to outbreaks.

In the eyes of families suffering from immunosuppressive conditions— HIV, and other types of leukemia, sarcoma and lymphoma, among them— parents who choose not to take advantage of immunizing their kids when they can are preventing measles from being exterminated in the U.S. and threatening their children in the process.

For the Waldrons, the fear of their son, Liam, catching a cold, let alone measles, has restricted when and where they go. When Liam’s white blood cell counts are low, they don’t leave the house. When they go to public places, he wears a face mask to minimize potential exposure to an infection.

“I think everybody in general only thinks about their own children at one point or another,” Wendy Waldron said, “but when there’s an outbreak like this, and you’re making a decision not only for your child but for your family and other children if that child is going to school … you know, I can’t relate to it, and I don’t understand it. And I would say that whether I was in this situation or not.”

“No one wakes up one day and wants to find out that their child has cancer, so we didn’t even have an opportunity to get him immunized,” she said. “I didn’t even have a choice.”