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

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

itoggle caption Scott Olson/Getty Images

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

California is one of 20 states that allow parents to opt out of vaccination requirements for reasons of "personal belief." About 10 percent of students in the Santa Monica-Malibu school district are not immunized. Justin Sullivan/Getty Images hide caption

itoggle caption Justin Sullivan/Getty Images

Dr. Eric Ball examines a healthy 5-day-old patient in his office in Ladera Ranch, Calif. Ball and colleagues decided this week to take only patients whose parents follow the recommended vaccine schedule. Courtesy of Eric Ball hide caption

itoggle caption Courtesy of Eric Ball

Jackie Carnegie immunizes Mabel Haywood in a Colorado Health Department immunization van in 1972. Shots for measles and other infectious diseases were offered. Ira Gay Sealy/Denver Post Archive/Getty Images hide caption

itoggle caption Ira Gay Sealy/Denver Post Archive/Getty Images

Writer Roald Dahl and his wife, actress Patricia Neal, with two of their children, Theo and Chantel Sophia "Tessa." The photo was taken a few years after oldest daughter, Olivia, died of measles. Hulton Archive/Getty Images hide caption

itoggle caption Hulton Archive/Getty Images