I live in Mississippi and I have a lung disease; my condition is getting worse and my physician is advising me to move to a home that is one story. The current home I am renting is two stories and I do not believe the landlord will be willing to let me out the lease. With the physician documentation, do I have a case to break the lease?

No. Sorry, unless the actual lease has this in writing. Options moving bed downstairs or other changes. You can talk landlord , ask, see if together find new renters

I'm guessing the physician's advice had to do with your not climbing stairs. If so, that is unlikely to allow you out of your lease.

How much time remains on the lease?

I see you have several options;

1) Move your bedroom downstairs so you don't have to climb
2) Install a chair rail to take you up and down (these can be rented and may be covered by your health insurance if your doctor prescribes it)
3) Talk to the landlord and see if he will release you or agree to a buyout.
4) Find someone to sublet (if the lease allows)
5) Find a replacement tenant he will agree to