It only takes one.

Reported by many media outlets today the Centers for Disease Control and Prevention said that "the biggest U.S. measles outbreak in a decade...was traced back to a 17-year-old girl who had traveled to Romania without first getting vaccinated." The other patients "became infected after they attended a church gathering with her the day after her return," and "only two of the 34 people had been vaccinated against measles." Again, we see how hard it is to get everyone vaccinated against a virus that spreads so easily. Herd immunity for measles requires near 100% immunization of he population.