BAKERSFIELD, Calif. -- In 2008, the United States had its biggest measles outbreak in more than 10 years, and nearly all cases were in people who were not vaccinated.

Health officials fear we will be seeing more of these outbreaks because parents are increasingly opting out of vaccines for their children.

Researchers from Emory University reviewed the current situation and found some disturbing trends.

Many states are making it easier for children to attend school without being immunized. States that allow "personal belief" exemptions have seen a doubling in the percentages of kids who are not vaccinated.

In certain U.S. counties, as many as 1 in 5 kids are now exempted from childhood shots. These areas are now vulnerable to infectious diseases, which can spread easily among under-vaccinated populations.

One study found children who were not vaccinated were 35 times more likely to contract measles.

Researchers said parents overestimate the side effects of shots and underestimate the harm from infectious diseases. Before a vaccine existed, measles killed 450 Americans and caused 4,000 cases of brain inflammation each year