ST. PETERSBURG, Fla. – With measles cases popping up all over the country--including seven reported cases in Pinellas County--the country is facing its biggest outbreak in years.

The measles vaccine is extremely effective, but vaccination rates have been dropping which has threatened the “herd immunity” of our communities.

MORE: Four more measles cases in Pinellas County's first outbreak in 20 years

In a nutshell, herd immunity is how we keep a preventable disease from spreading throughout our population.

Germs can spread quickly through a group of people who haven’t been immunized.

If the group has herd immunity, not only are the immunized people protected – but those people also protect certain high-risk people in the group like infants or cancer patients who can't get vaccines.

Those high-risk people rely on herd immunity to keep them safe from infectious disease.

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