Immunizations, like any medication, can cause side effects. However, a decision not to immunize a child also involves risk. It is a decision to put
the child and others who come into contact with him or her at risk of contracting a disease that could be dangerous or deadly. Consider measles. One out of 17 children with measles gets pneumonia. For every 1,000 children who get the disease, one or two will die from it. Thanks to vaccines, we have few cases of measles in the U.S. today. However, the disease is extremely contagious, and each year dozens of cases are imported from abroad into the U.S., threatening the health of people who have not been vaccinated and those for whom the vaccine was not effective. Unvaccinated children are also at risk from meningitis (swelling of the lining of the brain) caused by Hib (a severe bacterial infection), bloodstream infections caused by pneumococcus, deafness caused by mumps, and liver cancer caused by hepatitis B virus.
http://www.cdc.gov/vaccines/pubs/dow.../f_vacsafe.pdf