Over the course of one summer vacation, Tyler Ludlum went from being a healthy 10-year-old, looking forward to the pool, to an emotionally and physically traumatized preteen who'd traded both of his feet, and half the fingers on his right hand, for his life.

It could have been prevented if he – or those around him – had been vaccinated.

Tyler had contracted meningococcal meningitis, a swelling of membranes around the brain and spinal cord that's caused by bacteria passed by nasal or oral droplets. Tyler was likely in the vicinity of a perhaps asymptomatic carrier of the disease, when this person coughed or sneezed.

But his story is more than a case of being in the wrong place at the wrong time: A vaccine that's at least 85 percent effective at preventing meningococcal meningitis is widely available and strongly recommended by health officials. Too young to have received the vaccination as part of a routine visit, Tyler was dependent on those around him to be immunized.

"It is the worst thing on Earth to watch your child's tissues slowly dying in front of you," Tyler's mother Shara Ludlum told LiveScience. Now a participant in the Voices of Meningitis public education campaign, she hopes her son's story will encourage others to get vaccinated. (Tyler, who is now 12 years old, has two prosthetics that run from the bottom of his leg bones to just under his knees, along with removable feet that he can add J-shaped springs to for running. Luckily, he escaped suffering brain damage.)

And yet, some parents are choosing to avoid routine immunizations for fear they are putting their child in harm's way. It's a decision that puts their kids – and their communities – at risk, experts say.

Fear of vaccination “is a reasonable gut reaction," said Paul Offit, chief of the Division of Infectious Diseases and the director of the Vaccine Education Center at the Children's Hospital of Philadelphia. But it’s a fear that should be battled with the knowledge offered by science, Offit added.

No one likes seeing a child hurt, and that discomfort only escalates when watching a vial of strange liquid enter a baby's thigh through a vaccination needle. Pair that disturbing sight with a mysterious illness that happens to occur soon after, and they may seem to be linked, Offit said.

The Internet teems with rumors and anecdotes about links between vaccines and devastating illnesses. The stories online are usually heartbreaking: My happy, sociable child developed digestive issues in the weeks following immunization, which have since led to autism; hours after getting her vaccinated, our healthy little girl started showing symptoms…

However, just because two things happened around the same time, does not mean one caused the other, Offit said.

Still, increasing numbers of parents are falling prey to the rumors and choosing to eschew vaccinations, thinking that they are playing it safe. [5 Dangerous Vaccination Myths]

"But the choice to not get a vaccine is not a risk-free choice," Offit said. "A lot of diseases are out there, and if you choose to let your guard down, your child could suffer."

When vaccination rates decline

The number of children in private health plans getting properly immunized declined by as much as 3.5 percent last year, according to a recent report by the National Committee on Quality Assurance (NCQA), a nonprofit organization that tracks health care quality.

While overall immunization rates remain high, any drop is a cause for concern, said Jeffrey Dimond of the Centers for Disease Control and Prevention (CDC). "When a drop like that happens, there is often a corresponding rise in the diseases vaccines are supposed to protect us from," he told LiveScience.

Diseases such as polio and mumps, while rarely seen in the United States, have not been eradicated. And when contracted, they can have devastating consequences.

Parents who have watched their child suffer brain damage, become paralyzed, go into a coma or die after contracting the flu, measles, or some other disease for which vaccinations exist are always stunned that it happened to them, Offit said.

In pockets around the country where vaccinations are routinely shunned, the risk of infectious disease can be high. In 2006 and 2009, there were outbreaks of mumps that sickened thousands in the Midwest and East Coast, respectively. In 2008, the United States saw its biggest measles epidemic in more than a decade. And the current outbreak of whooping cough is far exceeding its usual cyclical increase, Offit said.

"It is not okay to say 'It is my right to catch a transferrable deadly disease,'" he said, because the decision to avoid vaccination affects an entire population.

Protected by the herd

It is possible, although unlikely, to contract an infectious disease even if properly immunized. That chance increases if portions – say, 15 to 30 percent – of one's community haven't received vaccinations. "Opting out" on this scale makes it difficult to achieve "herd immunity," where the whole population is considered safe from sporadic outbreaks because a critical percentage has been vaccinated.

With polio, a vaccination rate of 70 percent is enough to reach herd immunity. But for something as infectious as measles, unless the rate reaches 80 percent or more, significant outbreaks can occur, Offit said.

"We are now getting past the tipping point," said Offit, where communities are no longer protected by herd immunity against many infectious diseases.

While some diseases have vaccination rates that top 90 percent, only about 75 percent of children 19 to 35 months of age were fully immunized in 2007, reports the NCQA. The vaccination coverage for meningococcal meningitis – the disease that took Tyler's feet – is estimated at around 50 percent. The vaccine is approved for children 2 years and older, but is usually not given until age 11 because teens and preteens are most at risk. Like others under age 11, Tyler was reliant on the immunity of others.

Herd immunity may have also failed nine babies in California this year.

The state's current outbreak of pertussis, or whooping cough, which has sickened more than 6,000 people in the state since Jan. 1, is affecting an area where people are known to refuse vaccines, Offit said. Of the 10 who have died, nine were reportedly too young to have been vaccinated.

Snubbing the safety net

In many cases, a routine vaccination is missed due to ignorance or inadequate health care. Vaccination rates for children on Medicaid hover below those of private health plans by several percentage points, according to the NCQA.

But some parents are purposely avoiding vaccinations, opting for a "philosophical exemption" to excuse their child from school immunization requirements, Dimond said. They point to rumors littering the Internet that suggest shots can make children autistic or otherwise chronically ill.

Diseases like sudden infant death syndrome (SIDS), autism, ADHD, multiple sclerosis and cancer — where the causes are complex and mostly unknown — are prone to attempts at explanation. It is human nature to want to find clear, controllable triggers for such tragedies, especially when they occur in children.

But scientific studies have consistently shown such rumors to be unfounded. As heartbreaking as they are, emotional anecdotes must be analyzed in the light offered by science, Offit said.

The science

Large-scale studies on the topic have found that kids who are vaccinated are not more prone to chronic diseases than kids who are not vaccinated. Despite ample effort to find scientific evidence that would support the rumors and anecdotes, scientists have found no correlation between vaccinations and autism, other neurological or developmental problems, SIDS, multiple sclerosis, diabetes and other autoimmune diseases (with the possible exception of little-understood and exceedingly-rare Guillain-Barré syndrome).

Still, the library of medical literature is a vast and varied place, and one can find a few theories put forth by scientists that suggest ways a vaccine could make a kid sick.

These are among the more-compelling studies pointed to by Barbara Loe Fisher, co-founder of the National Vaccine Information Center (NVIC), a non-profit organization that held a conference on vaccinations in Reston, Va., last month. Fisher, who describes her oldest son as having vaccine-induced learning difficulties, is a prolific author, speaker and champion of the anti-vaccine movement.

However, since vaccinated kids are not more likely to be chronically ill than other kids, theories about possible mechanisms for vaccination-induced damage are little more than interesting academic exercises.

For instance, one review article singled out by Fisher, from the Israel Medical Association Journal, cites a study that found the Hemophilus influenzae type B (HiB) vaccination, which protects against the leading cause of bacterial meningitis in infants, stimulates the immune system. The authors theorize that a cascade of reactions following such a stimulation could, possibly, make a child more at risk for developing type I diabetes. However, the same review article also points to two large-scale studies that followed vaccinated and unvaccinated children for 10 years and found no association between the HiB vaccine and type I diabetes. The largest, published in the New England Journal of Medicine in 2004, followed 739,694 children and found no correlation between diabetes and any vaccination.

If vaccinations caused illnesses, scientists explain, large-scale studies would find positive correlations between vaccines and illness. But such studies consistently find no correlation.

There is, however, an overwhelmingly clear link between being unvaccinated and the likelihood of contracting an infectious disease. For example, more than 90 percent of people not immunized against measles will develop the infection if they are exposed to it.

Vaccines "better tested" than antibiotics, vitamins

"Vaccines are the safest, best-tested things we put in our bodies," Offit said. "They are better tested than antibiotics, better than vitamins, certainly better than nutraceuticals."

What should a parent worry about when considering immunizations? Shots will make an infant cry, even inconsolably in some rare cases. For older kids, the anticipation of pain can occasionally cause fainting, Dimond said. Soreness at the injection site or a fever can also develop.

"Nothing is completely safe," Offit said, "but the actual side effects are not what people worry about."

Parents should also be concerned about keeping their own vaccinations up-to-date, if not for their own sake, then to protect their children. The CDC recommends all adults receive flu shots yearly and booster shots against diseases such as whooping cough every 10 years.

When Shara Ludlum hears of parents making decisions about vaccines, she asks, "Why not prevent the worst outcome from happening?"