Map: Where California's unvaccinated people live

As the country watches the spread of a measles outbreak, eyes have trained on California. The recent flare-up originated at the Disneyland amusement park outside Los Angeles, and health authorities have said the disease is almost solely affecting unvaccinated people because the existing vaccine is highly effective.

This comes as health officials in the Golden State have been battling what they say are the worst outbreaks of pertussis—also known as whooping cough—in 70 years.

Like-minded people tend to cluster geographically, and people's views will often change to reflect those of the people with whom they interact most often. This clustering of unvaccinated people is of particular concern to health authorities, especially when it comes to the question of protecting infants too young to receive vaccines, or those medically unable to receive them.

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"We protect babies in the community by achieving very high vaccination rates and hoping they don't get exposed, but that concept of herd protection breaks down at the local level if you have like-minded unvaccinated families living together," Dr. Jane Seward of the CDC told CNBC in an interview last week. "All their babies are going to be exposed if measles comes in."

Vials of measles, mumps and rubella vaccine are displayed on a counter at a Walgreens Pharmacy on January 26, 2015 in Mill Valley, California.
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Vials of measles, mumps and rubella vaccine are displayed on a counter at a Walgreens Pharmacy on January 26, 2015 in Mill Valley, California.

In California, as in other states, many of the least vaccinated communities are relatively affluent and educated. And not all areas with relatively low vaccination rates are populated by "anti-vaxxers," or people who adamantly oppose vaccines for ideological or political reasons.

"There is this phenomenon where otherwise educated people worry that there is some kind of conspiracy, that ... the government is in cahoots with the pharmaceutical industry to make money selling vaccines, that there are these negative effects of vaccines," said Dr. Arthur Reingold, of UC Berkeley's School of Public Health. "I suppose there is also a background of mistrust of authority and government. But there is a greatly exaggerated fear of the risks of vaccines, and people want to do what they think is best for themselves and their children."

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And pockets with low vaccination rates exist in many places in the United States besides California.

"I think almost every state could point to some areas that are unvaccinated," Seward said. Recent outbreaks of measles have occurred in Somali communities in Minneapolis, among Orthodox Jews in New York, the Amish in Ohio, and in many other regions of the country.

Oregon and Washington also have areas where as much as 20 percent of the population may be unvaccinated.