It doesn't take a huge unpaid medical bill to make a collection agency come calling ... and calling.

Researchers found in a study of credit reports that more than 2 percent of adults had medical bills under $200 sent to a collection agency. Over half of the annual medical collections were for less than $600, according to the study, which examined 2016 credit reports for more than 4 million unidentified people.

"A lot of these bills are much more modest than many people will think," said one of the researchers, Benedic Ippolito, an economist with the think tank American Enterprise Institute.

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