Researchers used online data to model the vaccination rate in communities affected by an outbreak of mumps in Arkansas. Maimuna Majumder/HealthMap; Alyson Hurt/NPR hide caption

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Maimuna Majumder/HealthMap; Alyson Hurt/NPR

The mumps virus is on the loose. Cases are popping up in Arkansas, Iowa and Illinois. Alissa Eckert/CDC hide caption

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Alissa Eckert/CDC

Mumps Bump: Cases Rise In Iowa, Illinois And Arkansas

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Each dot represents one case of mumps between late June 2009 and late June 2010. The New England Journal of Medicine hide caption

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The New England Journal of Medicine

Two boys study together at a Chicago yeshiva in 2009. Public health officials say this type of close physical contact caused a mumps outbreak to spread throughout several orthodox Jewish communities in and around New York City. M. Spencer Green/AP hide caption

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M. Spencer Green/AP