I have a collection agency seeking payment from an old bill from over 3 years ago. Am I legally required to pay them?

I had insurance which paid a majority of my fees, however, I was soon unemployed and only able to pay off about $1000 of the $5000 bill. After missing a first payment, I was immediately sent to a collection agency. After building up my credit (815) and paying off credit card debt of $12,000 in the last year am I still legally obligated to pay this bill?