Vaccines save lives. It has been proven time and time again for diseases ranging from polio to measles to the flu. The more people a community has who are vaccinated, the healthier it is.

Those who remain unvaccinated leave themselves susceptible to serious infectious diseases. This puts everyone else at risk. That is why every state mandates that children receive vaccinations against a number of diseases, including polio and measles, as a condition of entering school.

If vaccination rates were to significantly decline, the United States could face a public health crisis.

One important goal of vaccine mandates is to achieve "herd immunity" in a community. That occurs when the percentage of members who have been immunized is high enough that a disease cannot gain a foothold. This prevents disease outbreaks from occurring at all.

However, most state mandate laws contain exceptions. These cover children whose parents object on religious grounds and sometimes those who object based on general philosophical principles. Wide use of these exemptions can put everyone at risk. If enough parents use them, herd immunity is not achieved and disease outbreaks may occur.

The Supreme Court has held that the Constitution does not require states to exempt children from mandatory vaccination for religious or philosophical reasons. However, most states permit these opt-outs on the assumption that few enough parents will use them that herd immunity can still be achieved. But if this assumption turns out to be wrong, the threshold vaccination rate for herd immunity will not be met.

Forty-eight of the 50 states provide exemptions for those who claim their religion forbids vaccination. Eighteen states have also granted exemptions for individuals who claim non-religious cultural or philosophical objections to vaccines. In some states, these can be obtained by simply checking a box on a simple form.

This is where problems arise. For example, because of vaccination exemptions, the United States has experienced several outbreaks of measles. During the period between 1989 and 1991, there were 55,622 reported cases, mostly in children under 5 years of age, more than 11,000 hospitalizations, and 125 deaths. The increase in measles cases occurred because of a failure to vaccinate children between 12 and 15 months old.

While the magnitude of this outbreak doesn't compare to the 500,000 cases of measles reported each year prior to the development of a vaccine, those 125 individuals probably died unnecessarily. In contrast, by 1998 the vaccination had rate increased, and there were only 89 cases of measles in the United States with no measles-associated deaths. If there is an increase in individuals seeking exemptions from vaccine mandates, the progress we have made to halt the spread of this deadly infectious disease could be unwound.

Some parents fear that vaccines can cause autism and do not want to put their children at risk. However, studies have repeatedly shown that this is not true. This was confirmed by an extensive report and review of research published in 2004 by the Institute of Medicine.

Just last month, yet another study, this one published in the Journal of Pediatrics, again refuted the alleged vaccine-autism link. And the original findings that claimed an association between vaccines and autism were uncovered as an elaborate fraud.

In order to maintain herd immunity and protect all members of society, particularly those who are especially vulnerable because of weakened immune systems, we must continue to vaccinate our children. If states continue to offer religious exemptions, they must be stringent in ensuring that only those with genuine religious opposition qualify. And the eighteen states that permit exemptions for philosophical reasons should consider repealing them.

Please, vaccinate your children. Otherwise, you put your child, and the rest of society, at risk for an outbreak of life-threatening diseases.