Princeton University-led research found that measles, one of the world's most contagious diseases, can spread in schools quickly unless vaccination rates are very high. The researchers used data from a 1904 measles outbreak in London to conduct one of the first examinations of how measles spreads at the school level. The researchers developed a mathematical model (above) based on current vaccination estimates in California where 95 percent of students are vaccinated against measles (gray box). In a hypothetical class of 77 students, a measles outbreak with a very high transmission rate (yellow) -- similar to what the researchers found existed in schools in 1904 -- infects students despite a high vaccination rate. The number of students infected with measles (left bar) begins to steadily increase as the proportion of vaccinated children (top bar, left to right) falls below 100 percent. Vaccination levels of roughly 90 percent and 80 percent, respectively, are able to contain an outbreak with a medium (red) or low (blue) transmission rate.