( CNN ) For many kids , a trip to Disneyland is a dream come true . But for some of those kids and their families who visited `` the happiest place on Earth '' a few weeks ago , that dream has become a nightmare . In the past month , 36 people have come down with measles traced to an exposure at the theme park , including five employees of Disneyland . Prior to the introduction of a vaccine in 1967 , nearly every American got the measles ; but since 2000 , it has effectively been eradicated in this country , with the only sources of exposure being foreign visitors or Americans who traveled and brought it back . The threat is growing , however , because not enough people are getting vaccinated and even for those who have gotten vaccinated , the overall trend is a problem for all of us . Vaccine rates in the United States have been in steady decline since the late '90s . Seventeen states now have fewer than 90 % of children vaccinated for measles . Often times , like-minded people who have unvaccinated children tend to cluster in the same communities , thus creating pockets with very low vaccination rates . The reduction in vaccination rates reveals one of the quirks of vaccines ; they only protect a population if nearly everyone is vaccinated . This is the concept of herd immunity . The magic number for measles is 95 , which means if at least 95 % of people in a community are vaccinated , everyone is protected . This is because the chance of the virus finding the individuals in the group who have little or no protection is very low . If a vaccinated , and protected , person gets measles they may not even know it , but the virus will be stopped in its tracks by their immune response before they can make more people sick . However , the more poorly protected , or unprotected , people there are , the easier it is for the virus to find them , make them sick and spread . For those of you who are vaccinated and might be thinking you 're protected from infection even in communities with vaccination rates lower than 95 % , you still need to worry . Adults can be affected , too . In fact , 13 of the 18 confirmed cases of measles in Orange County were adults . While the measles vaccine is extremely effective -- the best we have -- effectiveness can reduce over time , leaving both children and adults vulnerable , even if they were vaccinated . Therefore , the risk of any one of us coming down with a completely preventable childhood disease like measles will increase if vaccination rates continue to decline . Disney officials have recognized the importance of vaccination . After learning of the exposure , they offered vaccination and immunity tests to their employees , according to a statement issued by the Walt Disney Parks and Resorts chief medical officer , Dr. Pamela Hymel . Of the five employees affected , three have been medically cleared to return to work , a spokesman said , and others are on paid leave until medically cleared . To be sure , there are valid medical reasons that some children ca n't be vaccinated , such as allergies to components of the vaccine , underlying diseases that compromise the immune system , or because they are simply too young . But these children vitally depend on herd immunity to protect them from childhood diseases . The unfortunate reality , however , is that more and more parents are choosing not to vaccinate their children for nonmedical reasons . Some refuse vaccines on the grounds of religious beliefs ; others refuse on the repeatedly disproved argument that vaccines contribute to autism . Their high-risk decision not to vaccinate endangers not only their children , but also those who ca n't receive the vaccines , and even those of us who have had the vaccine a long time ago , and depend on herd immunity . 5 things to know about measles . Regardless of the reason why parents choose not to vaccinate their children , it is important for the rest of us to realize they are making the choice for all of us , too . By not vaccinating their own children , they increase everyone else 's chance of getting a preventable childhood disease like measles , whooping cough or even polio . Just last week , a 25-day-old baby died of whooping cough , which , like measles , is also spreading unnecessarily in the United States due to the decrease in vaccine rates . Just like the drunk driver who makes a socially irresponsible decision that can endanger not only his life , but also the lives of the other drivers and passengers on the road , parents who choose not to vaccinate their children put everyone else at risk . We can each play a part in protecting children by making sure parents understand their responsibility to vaccinate their children and the potential consequences on all of society if they do n't . It 's a small world after all , and the actions of the few can , and do , affect the many .
A measles outbreak resulted after exposure at Disneyland ; 36 became ill. Cynthia Leifer : Fewer people getting vaccinated , raising risks for all of us .