In the United States, your child's daycare center, preschool, or school can require you to provide proof that your child has had certain diseases or been immunized against them. All public elementary schools require proof of vaccination for children entering school, and most private schools do, too. These requirements vary from state to state.
Three types of exemptions to vaccines are available: philosophical exemptions, available in 20 states; religious exemptions, available in 41 states; and medical exemptions – if a child has had, for example, an allergic reaction to a vaccine component – available in all states.
The definition of personal or philosophical objection varies from state to state, but typically parents must write a letter or fill out a form (or in some cases sign a waiver on the back of the standard health form) explaining that they have strong personal philosophical objections to vaccinating. (Not coincidentally, the states that permit these exemptions have higher rates of measles, chicken pox, and pertussis.)
State health agencies and schools require proof of immunization because most diseases that can be prevented by vaccine are transmitted more easily when children congregate, as they do in schools. In the late 1970s, school nurses, public healthcare providers, family physicians, and parents realized the potential for diseases to spread rapidly in schools and vigorously sought laws requiring that children be vaccinated before entering school.
As a result of parental and community pressure, an increasing number of states had begun to allow exemptions for personal and philosophical reasons. But this didn't mean that all daycare centers or schools were required to accept all children. A private center or school may refuse entry to an unvaccinated child.
More recently, in the face of growing outbreaks of whopping cough (pertussis) and measles, some states have begun the process of making it more difficult to obtain philosophical exemptions.
Note: Not all places require every shot. For example, some don't yet require the hepatitis B or chicken pox vaccine.