Measles is one of the most contagious diseases known. Over 90 percent of unimmunized people who are exposed to the measles virus will develop the disease. Before the vaccine era, measles was considered a rite of passage in childhood, with millions of cases in the United States every year. Of those millions of cases, thousands of people were admitted to the hospital due to complications, and an average of 450 people died every year in the 1950s.
More recently, there was a measles epidemic in the United States from 1989 to 1991, affecting fifty five thousand people. This epidemic occurred during an era when the United States had high-quality sanitation and excellent medical facilities, and there were still 132 deaths from complications related to measles. Since then, the recommendation has been to give two doses of measles vaccine to all children before they enter kindergarten.
If you think this epidemic is an anomaly, Japan repealed mandatory immunization laws in the 1990s and suffered an epidemic of two hundred thousand cases of measles in 2000, with almost a hundred deaths. Another outbreak involving hundreds of students occurred in Tokyo in 2007.
Finally, in 2008, the rate of measles cases in the United States almost quadrupled, going from an average of 63 a year to 131 cases in the first seven months of 2008. Almost every case could be linked to an imported case of measles. However, the significant rise in number of cases did not occur because of more imported cases but rather because the same number of imported cases spread more widely among unvaccinated children. Although the numbers were small, this is an example of how herd immunity can fail with a highly contagious disease.