A recent outbreak of measles in San Diego highlights the possible dangers when children do not get the vaccinations required to be admitted to school.

Nine of the 12 children who developed measles in San Diego had not been inoculated against the virus because their parents objected; three of the children were too young to receive vaccines. Parents may think of measles as a mild disorder, but it can cause pneumonia, inflammation of the brain, and even death in rare cases. (And in economically developing countries, measles still kills 2 million children a year.)

A common reason parents object to vaccinations is their mistaken belief that vaccines are responsible for autism and other serious medical disorders. These concerns arose because many vaccines once contained a preservative made from a form of mercury, which of course can be particularly harmful to children. Web sites and parents organizations sprung up that claimed these trace amounts of mercury were responsible for a recent increase seen in the number of autism cases. Many of these Web sites opposed to vaccination continue to promulgate misinformation about the dangers of vaccination.

I recently pointed out that overwhelming evidence has convinced medical experts that vaccination does not cause autism. And mercury is no longer used in the vast majority of vaccines, including the live-virus vaccine for measles, mumps, and rubella. Most researchers believe the increase in autism diagnosis is due to our better understanding of the disorder, along with increased reporting of cases.

Unfortunately, these concerns of parents were amplified recently by news that a federal vaccine court agreed to pay damages to the family of an autistic child in Georgia, even though the child had inherited a mitochondrial disorder. Mitochondria, the parts of a cell that convert nutrients into energy, are the principal energy source of the body's cells.

There is no evidence that autism is due to a mitochondrial disorder. Although medical experts at the trial testified that vaccinations did not worsen the child's condition, the court sided with the family — much to the consternation of vaccine experts everywhere.

All states in the U.S. have a legal requirement that children be properly immunized before attending school. However, each state also offers exemptions for medical reasons; 48 states allow exemptions for religious reasons; and 19 states, including California, allow personal-belief exemptions for unspecified non-medical reasons.

A 2006 report in the Journal of the American Medical Association (JAMA) found that the average exemption rate in states that allowed personal-belief exemptions rose from 1 percent in 1991 to 2.5 percent in 2004.

Children who do not get vaccinated not only may face the dangers of the disorder themselves but also increase the risk of infection among those too young to be vaccinated or who are in the unusual situation where a vaccine was ineffective.

In fact, the JAMA article reported an average 50-percent increase in the incidence of whooping cough (pertussis) in those states with policies that easily grant exemptions for personal beliefs.

The results of medical research are not nearly as certain as the proofs in mathematics. Nonetheless, it is hard for me to understand why so many people reject the best available medical evidence and instead believe unfounded claims on the Internet and elsewhere if those assertions are repeated often enough or loudly enough.