A nurse holds a young girl who was vaccinated at the kickoff of a national measles prevention campaign in Liberia. Jason Beaubien/NPR hide caption

itoggle caption Jason Beaubien/NPR

Worth a little pain? Back in 1990, a school boy got a measles shot in the U.K., and it turns out, he got more than protection against the measles. Photofusion/UIG via Getty Images hide caption

itoggle caption Photofusion/UIG via Getty Images

Health worker Jackie Carnegie delivers a rubella vaccine in Colorado in 1972. Ira Gay Sealy/Denver Post via Getty Images hide caption

itoggle caption Ira Gay Sealy/Denver Post via Getty Images

People who oppose repealing the personal belief exemption gathered outside California's Capitol in Sacramento on Wednesday. Pauline Bartolone/Capital Public Radio hide caption

itoggle caption Pauline Bartolone/Capital Public Radio

Measles is highly contagious, and it produces fever and rash in susceptible people who become infected. Hazel Appleton/Health Protection Agency Centre/Science Source hide caption

itoggle caption Hazel Appleton/Health Protection Agency Centre/Science Source

Measles vaccine isn't a part of most workplaces. Eric Risberg/AP hide caption

itoggle caption Eric Risberg/AP
Gary Waters/Getty Images/Ikon Images

This World Health Organization map shows the percent of the population vaccinated for measles in each country in 2013. Dark green is at least 90 percent. Light green is 80 to 89 percent. Orange is 50 to 79 percent. Red is less than 50 percent. Courtesy of WHO hide caption

itoggle caption Courtesy of WHO

The KinderCare Learning Center in Palatine, Ill., where five infants have been diagnosed with measles. Officials are trying to track down the source of the infection. Scott Olson/Getty Images hide caption

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Leah Russin, of Palo Alto, Calif., holds her son, Leo, 16 months, as she speaks Wednesday at a news conference in support of proposed state legislation that would require parents to vaccinate all school children. Rich Pedroncelli/AP hide caption

itoggle caption Rich Pedroncelli/AP