When you receive a medical bill, it is not usually reported to the credit bureau. Once you become past due, then the hospital will report your account as past due.
When medical bills are reported to the credit bureau, they can affect your credit score by decreasing it. Medical bills will show up as a collection account or as a bad debt.
The amount that your credit score is decreased will depend on other circumstances. If your credit file is 10 or 15 years old and you have a lot of other credit that is paid on time, the medical bill will not lower your score by much.
The exact amount that your score is lowered will vary from consumer to consumer. Other things that are considered are the type of credit accounts you have and how much debt you have.
If medical bills are reported to the credit bureau, they will remain on your file for seven years. The longer they are on your file, the less of an impact they have on your credit score.
If you have no other credit except a medical bill collection, your credit score will be seriously affected. The reduction could be significant.