The biggest U.S. outbreak of measles since 1997 has sickened 127 people in 15 states, most of whom were not vaccinated against the highly contagious viral illness, federal health officials said on Wednesday.
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States with cases, the CDC said, include Arizona, Arkansas, California, Georgia, Hawaii, Illinois, Louisiana, Michigan, Missouri, New York, New Mexico, Pennsylvania, Virginia, Wisconsin and Washington state, as well as Washington, D.C.
LINK: http://www.reuters.com/article/domesticNews/idUSN0943743120080709
According to the CDC these cases are in people that have not been vaccinated because of fear of possible side effects.
I think it's cause some parents in underserved areas just will not bring in their infants to be vaccinated for various reasons: no transportation, religious beliefs, superstitious beliefs, relocation, poor parenting, etc. So they get lost in the cracks. There was an outbreak of pertussis a few years back in Arizona, due to poor compliance with vaccinating children.