To avoid vaccinating your child, would you send your child to a measles party? To a chicken pox party? Some parents do because they distrust vaccines and want their children to acquire natural immunity– a trend that scares American public health officials. In most states, vaccination is mandatory, although it has always been possible to get an exemption for religious reasons. However, the steep growth in the number of “exempters” is due to parents who believe that the vaccines are actually harmful. In recent years 21 states have created "personal-belief" or exemptions which allow parents to keep their children unvaccinated if they don’t believe in the procedure.

Only an estimated 2 to 3 percent of the 3 million children who attend kindergarten annually are not vaccinated. Exempters tend to cluster in certain neighbourhoods. Exemption rates of 15% to 18% have been found in Ashland, Oregon, and Vashon, Washington. In California, where the statewide rate of non-vaccinated children is about 1.5%, some counties were as high as 19% of kindergartners.

Although world wide the number of measles cases dropped 68% between 2000 and 2006, thanks to vaccinations, there are occasional outbreaks, even in the US. There is one in San Diego at the moment, with 64 cases. All but one of the children had not been vaccinated, although some were too young.

Why do American parents object? Many are scared by unsubstantiated reports that vaccines sometimes cause autism. Experts complain about misinformation from the internet. Some object on religious grounds. One group in Virginia says that "forcing vaccination is a violation of human rights."

“The very success of immunizations has turned out to be an Achilles’ heel,” Dr Mark Sawyer, a California infectious disease specialist told the New York Times. “Most of these parents have never seen measles, and don’t realize it could be a bad disease so they turn their concerns to unfounded risks. ~ Washington Post, June 10; New York Times, Mar 21