A medical bill for an amount of approximately $100 CAN affect your credit score, for purposes of getting a credit card. I know this because I had a similar situation to you-- I moved across the country and never received that piece of forwarded mail.
The only reason I found out about it was because I decided to apply for a new credit card and was declined, solely for that reason. By then, it was 5 months late, so that it showed as seriously delinquent (over 90 days) on my credit report (FICO/Experian) and had been referred to a collections agency.
I promptly contacted the collections agency, paid the $80.00 debt and asked if it could be removed from my credit report. The credit reporting agency said that they would record it as paid in full but there was nothing further to be done. I called the original physician who had issued the bill, and their billing office could do nothing.
I applied again for a credit card about a year later. I was denied again, due to having a seriously delinquent past account, that $80, even though I had paid in full.
Different criteria are used for secured borrowing such as a mortgage or auto loan. It will take between five and seven years for your medical bill to drop off your credit report, once you have paid it. Do pay it now. It may not have any negative impact on your ability to get a mortgage though, as that is a secured loan.