Moreover, with international travel now widespread and common, it is easier than ever to spread these diseases from one population to another. Most of the diseases that vaccines now prevent are most easily transmitted when people gathered in groups, as in schools. State laws requiring immunization prevent serious outbreaks of certain infectious diseases among school-aged children, and through them, to the rest of the population. The availability of vaccines has drastically reduced the number of people who get infectious diseases along with the complications that these diseases can produce. Without vaccination, epidemics of vaccine-preventable diseases would return.
All states permit exemption of children from vaccination for specific medical reasons.
For example, children with compromised immune systems from illnesses such as cancer or AIDS may not be able to receive immunizations. Since they cannot be vaccinated, these children remain vulnerable to diseases such as polio, measles, mumps, pertussis, and diphtheria. They are dependent on their community to protect them against disease through vaccination of all medically-eligible individuals. Some states exempt children from vaccination due to the religious or philosophical beliefs of their parents. Since they do not receive vaccines, these children are not at risk from possible adverse reactions to vaccines.