As Dolly points out, bills themselves (whether from a plumber or a doctor) do not affect your credit score. It is only after the bills are unpaid (or not paid on time), and if they are then reported to the credit bureaus as derogs, that they could hurt your score.
The usual way is for the unpaid debt to be turned over to a collection agency. Also possible would be for you to have a credit account that you begin to become late on (like a CareCredit card). So basically the two ways are collections and lates. (Also possible would be for an unpaid account to appear as a chargeoff.)
FICO 8 ignores all collections where the original amount was under $100 (paid or unpaid). This is true for all three bureaus.
Models older than FICO 8 (such as your mortgage scores) penalize you for collections of any size, even if the collection is listed as paid.
The most recent model (FICO 9) ignores all paid collections of any size (medical or non-medical). I do not know whether FICO 9 ignores small unpaid collections (< $100) like FICO 8 does. I think we may have seen evidence here that suggests it does not ignore them. If you have an unpaid medical collection, it will hurt your score less under FICO 9 than a non-medical collection. So FICO 9 softens the blow but doesn't eliminate it.
As our legal guru RobertEG writes in a thread from March of this year:
"Additionally, be aware that getting medical debt paid by your insurance is the easiest and fastest way to get medical collections deleted.... The new CRA policy, announced under its "National Consumer Assistant Plan" in 2015, includes their own removal of any medical collection if it is shown that it was paid via a medical insuror. That eliminates any need for GW or PFD requests, or use of other processes, such as the so-called HIPAA process."
In REG's closing words, he alludes to a technique some people out on the web have looked into it that involves leveraging the HIPAA laws to help with medical debt removal. Due to the terms that govern discussions on the forum we do not discuss such approaches or endorse their use. Should a person want to find out more, such a person can easily find out more with a standard google search, e.g. medical debt collections HIPAA
