The price of disease prevention in the group may be injury to an occasional individual. The fact that polio vaccine prevents thousands of cases of paralytic polio is little comfort to the rare individual who gets polio from the vaccine. Most mandatory immunization laws contain exceptions for individuals who have a high probability of being injured by an immunization. Many of these laws also exempt persons who have religious objections to immunization. The U.S. Constitution allows mandatory immunization of religious objectors, but most states do not take advantage of this power.

The effectiveness of the immunization laws depends on compliance by physicians and parents. If physicians give medical exemptions to a large percentage of their patients, the level of immunity in their school system might drop low enough to support a disease epidemic. The physician might be liable for the results of the disease in any child he or she exempted improperly. The physician also might be liable for injuries to children who are not the physician’s patients who would not have been exposed to the disease but for the physician’s improper behavior.