You are supposed to get a notice telling you about medical debt before it ends up in collections. The medical care providers want to give you a chance to pay it off. They'd rather avoid collections if they can. However, it is possible to be contacted about money owed by a collections agency, even if you were never notified and if you didn't even know you had an outstanding bill.
The problem is that the system isn't perfect. If they had an old address for you, they may have sent multiple notices to the old house, notices that you never saw. A notice could have been lost in the mail, even if they had the right address. They may have called you after you changed phone lines, meaning you never got it.
When these things happen, that doesn't waive the debt. You still owe that money. In most cases, the medical care provider doesn't even know that you didn't get the notices. They just assume you're not responding and not paying, and they'll move on to collections if they have to.
If this happens, you could get an unexpected bill in the mail for something that happened months or even years ago. If it's already in collections, they'll likely be asking you to pay all of it off immediately, and your credit score could already be dropping. Even if you didn't intentionally do anything wrong, this can have a lasting impact on your financial situation.
When unexpected medial debt shows up in Pennsylvania, make sure you know about all of the legal options that you have available to you.
Source: Credit.com, "4 Medical Bill Myths That Can Cost You Dearly," Gerri Detweiler, accessed March 15, 2016